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/views/SearchView.jsx

@@ -5,6 +5,7 @@ import ProductResults from '../components/ProductResults';
 import PharmacyList from '../components/PharmacyList';
 import PharmacyMap from '../components/PharmacyMap';
 import { haversineKm, getUserPosition, hasCachedPosition } from '../utils/geo';
+import { useTranslation } from '../i18n';
 import './SearchView.css';
 
 const suggestions = [
@@ -15,6 +16,7 @@ const suggestions = [
 ];
 
 function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigateToProduct }) {
+  const { t } = useTranslation();
   const [searchQuery, setSearchQuery] = useState(initialQuery);
   const [medicines, setMedicines] = useState([]);
   const [products, setProducts] = useState([]);
@@ -193,11 +195,11 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
         console.error('Error saving search history:', err);
       }
     } catch (err) {
-      let msg = 'No se pudo obtener tu ubicación';
+      let msg = t('search.locationError');
       if (err && typeof err.code === 'number') {
-        if (err.code === 1) msg = 'Permiso de ubicación denegado. Permite el acceso a la ubicación en tu navegador.';
-        else if (err.code === 2) msg = 'Ubicación no disponible. Verifica que el GPS esté activado.';
-        else if (err.code === 3) msg = 'La ubicación tardó demasiado. Intenta de nuevo o verifica tu conexión.';
+        if (err.code === 1) msg = t('search.locationDenied');
+        else if (err.code === 2) msg = t('search.locationUnavailable');
+        else if (err.code === 3) msg = t('search.locationTimeout');
       }
       setLocationError(msg);
     } finally {
@@ -223,15 +225,15 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
         <SearchBar
           value={searchQuery}
           onChange={setSearchQuery}
-          placeholder="Escriba el nombre del medicamento"
+          placeholder={t('search.placeholder')}
         />
 
-        {loading && <div className="loading">Buscando...</div>}
+        {loading && <div className="loading">{t('search.searching')}</div>}
 
         {!searchQuery && !selectedMedicine && (
           <>
             <section className="suggestions-section">
-              <h2 className="section-title">Sugerencias</h2>
+              <h2 className="section-title">{t('search.suggestions')}</h2>
               <div className="suggestions-grid">
                 {suggestions.map((s, i) => (
                   <button
@@ -252,7 +254,7 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
 
             {currentUser && recentSearches.length > 0 && (
               <section className="recent-section">
-                <h2 className="section-title">Resultados Recientes</h2>
+                <h2 className="section-title">{t('search.recentResults')}</h2>
                 <div className="recent-list">
                   {recentSearches.map((r) => (
                     <div
@@ -285,7 +287,7 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
                           <circle cx="12" cy="12" r="10" />
                           <polyline points="12 6 12 12 16 14" />
                         </svg>
-                        Encontrar cerca
+                        {t('search.findNearby')}
                       </button>
                     </div>
                   ))}
@@ -299,7 +301,7 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
           <>
             <div className="results-summary">
               {(medicines.length + products.length) > 0 && (
-                <span>{medicines.length + products.length} resultados encontrados</span>
+                <span>{medicines.length + products.length} {t('search.resultsFound')}</span>
               )}
             </div>
 
@@ -318,7 +320,7 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
 
             {products.length > 0 && (
               <div className="products-section">
-                <h3 className="section-subtitle">Parafarmacia</h3>
+                <h3 className="section-subtitle">{t('search.parapharmacy')}</h3>
                 <ProductResults
                   products={products}
                   onSelect={(p) => {
@@ -340,9 +342,9 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
             <div className="medicine-info">
               <h2>{selectedMedicine.name}</h2>
               <div className="medicine-details">
-                <span><strong>Ingrediente Activo:</strong> {selectedMedicine.active_ingredient}</span>
-                <span><strong>Dosis:</strong> {selectedMedicine.dosage}</span>
-                <span><strong>Forma:</strong> {selectedMedicine.form}</span>
+                <span><strong>{t('search.activeIngredient')}</strong> {selectedMedicine.active_ingredient}</span>
+                <span><strong>{t('search.dosage')}</strong> {selectedMedicine.dosage}</span>
+                <span><strong>{t('search.form')}</strong> {selectedMedicine.form}</span>
               </div>
               <button
                 className="back-button"
@@ -351,7 +353,7 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
                   setPharmacies([]);
                 }}
               >
-                ← Volver a búsqueda
+                {t('search.backToSearch')}
               </button>
             </div>
 
@@ -363,24 +365,24 @@ function SearchView({ currentUser, onLoginRequest, initialQuery = '', onNavigate
                   disabled={locating}
                 >
                   {locating
-                    ? '📍 Localizando…'
+                    ? t('search.locating')
                     : sortByDistance
-                      ? '📍 Ordenado por distancia · Reset'
+                      ? t('search.sortedByDistance')
                       : hasSavedCoords
-                        ? '📍 Ordenar por ubicación guardada'
-                        : '📍 Ordenar por distancia'}
+                        ? t('search.sortBySavedLocation')
+                        : t('search.sortByDistance')}
                 </button>
                 {sortByDistance && positionSource === 'profile' && (
-                  <span className="location-source">Usando tu dirección guardada</span>
+                  <span className="location-source">{t('search.usingSavedAddress')}</span>
                 )}
                 {sortByDistance && positionSource === 'cached' && (
-                  <span className="location-source">Usando ubicación reciente</span>
+                  <span className="location-source">{t('search.usingRecentLocation')}</span>
                 )}
                 {locationError && (
                   <span className="location-error">
                     {locationError}
                     <button className="retry-location-btn" onClick={handleSortByDistance}>
-                      Reintentar
+                      {t('search.retry')}
                     </button>
                   </span>
                 )}