feat(i18n): add bilingual support (Català / Castellano)

Add lightweight i18n system using React Context + useTranslation hook.
No external dependencies. Language persisted in localStorage('ff-lang').

New files:
- src/i18n/locales/ca.js (~410 translation keys)
- src/i18n/locales/es.js (~410 translation keys)
- src/i18n/LanguageContext.jsx (Provider + t() function)
- src/i18n/useTranslation.js (hook)
- src/i18n/index.js (barrel export)

Modified 22 files:
- main.jsx: wrap App with LanguageProvider
- App.test.jsx: add LanguageProvider wrapper for tests
- 13 user components: BottomNav, HomeView, LoginModal, SearchView,
  ProfileView, AlertsView, ScannerView, PharmacyList, MedicineResults,
  ProductResults, SavedNotifications, ErrorBoundary, SearchBar
- 6 admin components: AdminView, LoginForm, PharmacyManagement,
  MedicineManagement, PharmacyMedicineLink, PharmacyProductLink

ProfileView includes language selector (globe icon + modal CA/ES)
placed next to the Theme selector.

No routes changed. No API calls affected. No navigation changes.

apps/frontend/src/components/admin/PharmacyMedicineLink.jsx

@@ -1,4 +1,5 @@
 import React, { useState, useEffect, useMemo, useRef } from 'react';
+import { useTranslation } from '../../i18n';
 import './AdminComponents.css';
 
 const MAX_PHARMACY_RESULTS = 25;
@@ -8,6 +9,7 @@ function normalize(s) {
 }
 
 function PharmacyMedicineLink() {
+  const { t } = useTranslation();
   const [pharmacies, setPharmacies] = useState([]);
   const [medicineSearch, setMedicineSearch] = useState('');
   const [medicineResults, setMedicineResults] = useState([]);
@@ -111,7 +113,7 @@ function PharmacyMedicineLink() {
     e.preventDefault();
     
     if (!selectedMedicine) {
-      alert('Por favor, selecciona un medicamento primero');
+      alert(t('admin.linkMedicine.selectMedicineFirst'));
       return;
     }
 
@@ -131,16 +133,16 @@ function PharmacyMedicineLink() {
         body: JSON.stringify(payload)
       });
 
-      if (!response.ok) throw new Error('Error al vincular medicamento a farmacia');
+      if (!response.ok) throw new Error(t('admin.linkMedicine.linkError'));
 
       resetForm();
       if (selectedPharmacy) {
         fetchPharmacyMedicines(selectedPharmacy.id);
       }
-      alert('¡Medicamento vinculado a la farmacia correctamente!');
+      alert(t('admin.linkMedicine.linkSuccess'));
     } catch (error) {
       console.error('Error linking medicine:', error);
-      alert('Error al vincular medicamento a farmacia');
+      alert(t('admin.linkMedicine.linkError'));
     }
   };
 
@@ -153,18 +155,18 @@ function PharmacyMedicineLink() {
         body: JSON.stringify({ price, stock })
       });
 
-      if (!response.ok) throw new Error('Error al actualizar');
+      if (!response.ok) throw new Error(t('admin.linkMedicine.updateError'));
 
       fetchPharmacyMedicines(selectedPharmacy.id);
-      alert('¡Actualizado correctamente!');
+      alert(t('admin.linkMedicine.updateSuccess'));
     } catch (error) {
       console.error('Error updating:', error);
-      alert('Error al actualizar');
+      alert(t('admin.linkMedicine.updateError'));
     }
   };
 
   const handleDelete = async (id) => {
-    if (!confirm('¿Eliminar este medicamento de la farmacia?')) return;
+    if (!confirm(t('admin.linkMedicine.deleteConfirm'))) return;
 
     try {
       const response = await fetch(`/api/admin/pharmacy-medicines/${id}`, {
@@ -172,13 +174,13 @@ function PharmacyMedicineLink() {
         credentials: 'include'
       });
 
-      if (!response.ok) throw new Error('Error al eliminar');
+      if (!response.ok) throw new Error(t('admin.linkMedicine.deleteError'));
 
       fetchPharmacyMedicines(selectedPharmacy.id);
-      alert('¡Medicamento eliminado de la farmacia!');
+      alert(t('admin.linkMedicine.deleteSuccess'));
     } catch (error) {
       console.error('Error deleting:', error);
-      alert('Error al eliminar medicamento');
+      alert(t('admin.linkMedicine.deleteError'));
     }
   };
 
@@ -216,11 +218,11 @@ function PharmacyMedicineLink() {
 
   return (
     <div className="admin-section">
-      <h2>Vincular Medicamento a Farmacia</h2>
+      <h2>{t('admin.linkMedicine.title')}</h2>
 
       <form className="admin-form" onSubmit={handleSubmit}>
         <div className="form-group">
-          <label>Farmacia *</label>
+          <label>{t('admin.linkMedicine.pharmacy')}</label>
           <input
             ref={pharmacyInputRef}
             type="text"
@@ -235,7 +237,7 @@ function PharmacyMedicineLink() {
             }}
             onFocus={() => setPharmacyDropdownOpen(true)}
             onBlur={() => setTimeout(() => setPharmacyDropdownOpen(false), 150)}
-            placeholder={pharmacies.length ? `Buscar entre ${pharmacies.length} farmacias por nombre o dirección…` : 'Cargando farmacias…'}
+            placeholder={pharmacies.length ? `${t('admin.linkMedicine.searchPharmacy')} ${pharmacies.length} ${t('admin.linkMedicine.pharmacies')}` : t('admin.linkMedicine.loadingPharmacies')}
             autoComplete="off"
             required={!selectedPharmacy}
           />
@@ -243,7 +245,7 @@ function PharmacyMedicineLink() {
             <div className="medicine-search-results">
               {filteredPharmacies.length === 0 ? (
                 <div className="search-result-item search-result-item--empty">
-                  <span>No hay farmacias que coincidan con "{pharmacyQuery}"</span>
+                  <span>{t('admin.linkMedicine.noPharmacies')} "{pharmacyQuery}"</span>
                 </div>
               ) : (
                 filteredPharmacies.map((pharmacy) => (
@@ -264,14 +266,14 @@ function PharmacyMedicineLink() {
               <p>✅ Selected: <strong>{selectedPharmacy.name}</strong></p>
               <p className="medicine-details">{selectedPharmacy.address}</p>
               <button type="button" className="btn-small" onClick={clearPharmacy}>
-                Cambiar farmacia
+                {t('admin.linkMedicine.changePharmacy')}
               </button>
             </div>
           )}
         </div>
 
         <div className="form-group">
-          <label>Buscar Medicamento (API CIMA) *</label>
+          <label>{t('admin.linkMedicine.medicine')}</label>
           <input
             type="text"
             value={medicineSearch}
@@ -279,10 +281,10 @@ function PharmacyMedicineLink() {
               setMedicineSearch(e.target.value);
               setSelectedMedicine(null);
             }}
-            placeholder="Escribe para buscar medicamentos en CIMA..."
+            placeholder={t('admin.linkMedicine.searchMedicine')}
             required
           />
-          {searching && <p className="loading-text">Buscando...</p>}
+          {searching && <p className="loading-text">{t('admin.linkMedicine.searching')}</p>}
           
           {medicineResults.length > 0 && !selectedMedicine && (
             <div className="medicine-search-results">
@@ -304,9 +306,9 @@ function PharmacyMedicineLink() {
             <div className="selected-medicine-info">
               <p>✅ Selected: <strong>{selectedMedicine.name}</strong></p>
               <p className="medicine-details">
-                {selectedMedicine.active_ingredient && `Principio activo: ${selectedMedicine.active_ingredient} • `}
-                {selectedMedicine.dosage && `Dosis: ${selectedMedicine.dosage} • `}
-                Nº Registro: {selectedMedicine.nregistro}
+                {selectedMedicine.active_ingredient && `${t('admin.linkMedicine.activeIngredient')} ${selectedMedicine.active_ingredient} • `}
+                {selectedMedicine.dosage && `${t('admin.linkMedicine.dosage')} ${selectedMedicine.dosage} • `}
+                {t('admin.linkMedicine.registrationNumber')} {selectedMedicine.nregistro}
               </p>
               <button 
                 type="button" 
@@ -316,7 +318,7 @@ function PharmacyMedicineLink() {
                   setMedicineSearch('');
                 }}
               >
-                Cambiar medicamento
+                {t('admin.linkMedicine.changeMedicine')}
               </button>
             </div>
           )}
@@ -324,7 +326,7 @@ function PharmacyMedicineLink() {
 
         <div className="form-row">
           <div className="form-group">
-            <label>Precio (€)</label>
+            <label>{t('admin.linkMedicine.price')}</label>
             <input
               type="number"
               step="0.01"
@@ -335,7 +337,7 @@ function PharmacyMedicineLink() {
           </div>
 
           <div className="form-group">
-            <label>Stock</label>
+            <label>{t('admin.linkMedicine.stock')}</label>
             <input
               type="number"
               value={formData.stock}
@@ -347,21 +349,21 @@ function PharmacyMedicineLink() {
 
         <div className="form-actions">
           <button type="submit" className="btn-primary">
-            Vincular Medicamento
+            {t('admin.linkMedicine.linkBtn')}
           </button>
           <button type="button" className="btn-secondary" onClick={resetForm}>
-            Reiniciar
+            {t('admin.linkMedicine.reset')}
           </button>
         </div>
       </form>
 
       {selectedPharmacy && (
         <div className="pharmacy-medicines-section">
-          <h3>Medicamentos en {selectedPharmacy.name}</h3>
+          <h3>{t('admin.linkMedicine.medicationsIn')} {selectedPharmacy.name}</h3>
           {loading ? (
-            <div className="loading">Cargando...</div>
+            <div className="loading">{t('admin.linkMedicine.loading')}</div>
           ) : pharmacyMedicines.length === 0 ? (
-            <p className="empty-state">Aún no hay medicamentos vinculados a esta farmacia.</p>
+            <p className="empty-state">{t('admin.linkMedicine.empty')}</p>
           ) : (
             <div className="admin-list">
               {pharmacyMedicines.map((pm) => (
@@ -369,7 +371,7 @@ function PharmacyMedicineLink() {
                   <div className="item-content">
                     <h4>{pm.medicine_name}</h4>
                     <p>
-                      <strong>Precio:</strong> {pm.price ? `€${parseFloat(pm.price).toFixed(2)}` : 'No definido'} • 
+                      <strong>{t('admin.linkMedicine.priceLabel')}</strong> {pm.price ? `€${parseFloat(pm.price).toFixed(2)}` : t('admin.linkMedicine.undefined')} •
                       <strong> Stock:</strong> {pm.stock || 0}
                     </p>
                   </div>
@@ -377,17 +379,17 @@ function PharmacyMedicineLink() {
                     <button 
                       className="btn-edit"
                       onClick={() => {
-                        const newPrice = prompt('Introduce nuevo precio:', pm.price || '');
-                        const newStock = prompt('Introduce nuevo stock:', pm.stock || '0');
+                        const newPrice = prompt(t('admin.linkMedicine.newPrice'), pm.price || '');
+                        const newStock = prompt(t('admin.linkMedicine.newStock'), pm.stock || '0');
                         if (newPrice !== null && newStock !== null) {
                           handleUpdate(pm.id, newPrice ? parseFloat(newPrice) : null, parseInt(newStock) || 0);
                         }
                       }}
                     >
-                      Actualizar
+                      {t('admin.linkMedicine.update')}
                     </button>
                     <button className="btn-delete" onClick={() => handleDelete(pm.id)}>
-                      Eliminar
+                      {t('admin.linkMedicine.delete')}
                     </button>
                   </div>
                 </div>