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/SavedNotifications.jsx

@@ -1,7 +1,9 @@
 import React, { useEffect, useState } from 'react';
+import { useTranslation } from '../i18n';
 import './SavedNotifications.css';
 
 function SavedNotifications({ onClose, onNotificationChange }) {
+  const { t } = useTranslation();
   const [loading, setLoading] = useState(true);
   const [error, setError] = useState(null);
   const [items, setItems] = useState([]);
@@ -21,7 +23,7 @@ function SavedNotifications({ onClose, onNotificationChange }) {
         ].sort((a, b) => (b.created_at || '').localeCompare(a.created_at || ''));
         setItems(merged);
       } catch (err) {
-        if (!cancelled) setError(err.message || 'No se pudieron cargar las notificaciones guardadas');
+        if (!cancelled) setError(err.message || t('savedNotifications.loadError'));
       } finally {
         if (!cancelled) setLoading(false);
       }
@@ -43,7 +45,7 @@ function SavedNotifications({ onClose, onNotificationChange }) {
       setItems(prev => prev.filter(i => !(i.scope === item.scope && i.id === item.id)));
       onNotificationChange?.();
     } catch (err) {
-      setError(err.message || 'No se pudo eliminar la notificación');
+      setError(err.message || t('savedNotifications.deleteError'));
     } finally {
       setBusyId(null);
     }
@@ -53,15 +55,15 @@ function SavedNotifications({ onClose, onNotificationChange }) {
     <div className="saved-notifications-backdrop" onClick={onClose}>
       <div className="saved-notifications-modal" onClick={e => e.stopPropagation()}>
         <div className="saved-notifications-header">
-          <h2>🔔 Notificaciones Guardadas</h2>
-          <button className="saved-notifications-close" onClick={onClose} aria-label="Cerrar">×</button>
+          <h2>{t('savedNotifications.title')}</h2>
+          <button className="saved-notifications-close" onClick={onClose} aria-label={t('savedNotifications.close')}>×</button>
         </div>
         <div className="saved-notifications-body">
-          {loading && <p className="saved-notifications-status">Cargando…</p>}
+          {loading && <p className="saved-notifications-status">{t('savedNotifications.loading')}</p>}
           {!loading && error && <p className="saved-notifications-error">{error}</p>}
           {!loading && !error && items.length === 0 && (
             <p className="saved-notifications-empty">
-              Aún no hay notificaciones guardadas. Toca la campana 🔕 en una farmacia sin stock para recibir notificaciones cuando se reponga.
+              {t('savedNotifications.empty')}
             </p>
           )}
           {!loading && !error && items.length > 0 && (
@@ -78,7 +80,7 @@ function SavedNotifications({ onClose, onNotificationChange }) {
                         {item.scope === 'pharmacy' ? (
                           <>
                             <span className="saved-notifications-chip saved-notifications-chip--pharmacy">
-                              🏥 {item.pharmacy_name || `Farmacia #${item.pharmacy_id}`}
+                              🏥 {item.pharmacy_name || `${t('savedNotifications.anyPharmacy')} #${item.pharmacy_id}`}
                             </span>
                             {item.pharmacy_address && (
                               <span className="saved-notifications-address">{item.pharmacy_address}</span>
@@ -86,7 +88,7 @@ function SavedNotifications({ onClose, onNotificationChange }) {
                           </>
                         ) : (
                           <span className="saved-notifications-chip">
-                            Cualquier farmacia
+                            {t('savedNotifications.anyPharmacy')}
                           </span>
                         )}
                       </div>
@@ -96,9 +98,9 @@ function SavedNotifications({ onClose, onNotificationChange }) {
                       className="saved-notifications-remove"
                       onClick={() => handleDelete(item)}
                       disabled={busyId === key}
-                      aria-label="Eliminar notificación"
+                      aria-label={t('savedNotifications.delete')}
                     >
-                      {busyId === key ? '…' : 'Eliminar'}
+                      {busyId === key ? '…' : t('savedNotifications.delete')}
                     </button>
                   </li>
                 );