feat(i18n-mobile): add bilingual support (Català / Castellano) for React Native app

Add lightweight i18n system matching the web app architecture.
Language persisted in AsyncStorage('ff-lang').

New files:
- src/i18n/locales/ca.js (~190 translation keys)
- src/i18n/locales/es.js (~190 translation keys)
- src/i18n/LanguageContext.jsx (Provider + AsyncStorage)
- src/i18n/useTranslation.js (hook)
- src/i18n/index.js (barrel export)

Modified 16 files:
- app/_layout.tsx: wrap with LanguageProvider, translate stack titles
- app/(tabs)/_layout.tsx: translate tab bar labels
- app/(tabs)/index.tsx: translate home screen
- app/(tabs)/search.tsx: translate search screen
- app/(tabs)/alerts.tsx: translate alerts screen
- app/(tabs)/profile.tsx: translate profile + add language selector (CA/ES)
- app/(tabs)/scan.tsx: translate scanner screen
- app/auth/login.tsx: translate login/register
- app/medicine/[id].tsx: translate medicine detail
- app/pharmacy/[id].tsx: translate pharmacy detail
- app/product/[source]/[id].tsx: translate product detail
- 5 components: MedicineCard, StockBadge, SearchBar, LoadingSpinner, BarcodeScanner

Language selector in Profile screen (globe icon + CA/ES toggle).
No routes changed. No API calls affected.

apps/frontend-mobile/app/(tabs)/search.tsx

@@ -14,6 +14,7 @@ import { useThemeContext } from '../../components/ThemeProvider';
 import { spacing, borderRadius } from '../../constants/theme';
 import { Medicine } from '../../types';
 import { config } from '../../constants/config';
+import { useTranslation } from '../../src/i18n';
 
 const TABLET_MIN_WIDTH = 768;
 
@@ -25,6 +26,7 @@ const suggestions = [
 ];
 
 export default function SearchScreen() {
+  const { t } = useTranslation();
   const { width } = useWindowDimensions();
   const isTablet = width >= TABLET_MIN_WIDTH;
   const { isAuthenticated } = useAuth();
@@ -68,7 +70,7 @@ export default function SearchScreen() {
         const data = await searchMedicines(debouncedQuery);
         setResults(data);
       } catch (err) {
-        setError('Error al buscar medicamentos');
+        setError(t('search.error'));
         console.error(err);
       } finally {
         setIsLoading(false);
@@ -106,7 +108,7 @@ export default function SearchScreen() {
       {query.length >= 2 && (results.length + products.length) > 0 && (
         <View style={styles.resultsSummary}>
           <Text style={[styles.resultsSummaryText, { color: colors.textSecondary }]}>
-            {results.length + products.length} resultados encontrados
+            {results.length + products.length} {t('search.resultsFound')}
           </Text>
         </View>
       )}
@@ -114,7 +116,7 @@ export default function SearchScreen() {
       {showSuggestions && (
         <>
           <View style={[styles.suggestionsSection, isTablet && styles.suggestionsSectionTablet]}>
-            <Text style={[styles.sectionTitle, { color: colors.text }]}>Sugerencias</Text>
+            <Text style={[styles.sectionTitle, { color: colors.text }]}>{t('search.suggestions')}</Text>
             <View style={styles.suggestionsGrid}>
               {suggestions.map((s) => (
                 <TouchableOpacity
@@ -134,7 +136,7 @@ export default function SearchScreen() {
 
           {isAuthenticated && recentSearches.length > 0 && (
             <View style={[styles.recentSection, isTablet && styles.recentSectionTablet]}>
-              <Text style={[styles.sectionTitle, { color: colors.text }]}>Búsquedas recientes</Text>
+              <Text style={[styles.sectionTitle, { color: colors.text }]}>{t('search.recentSearches')}</Text>
               {recentSearches.map((term) => (
                 <TouchableOpacity
                   key={term}
@@ -154,7 +156,7 @@ export default function SearchScreen() {
         </>
       )}
 
-      {isLoading && <LoadingSpinner message="Buscando..." />}
+      {isLoading && <LoadingSpinner message={t('search.loading')} />}
 
       {error && (
         <View style={[styles.errorContainer, isTablet && styles.errorContainerTablet, { backgroundColor: colors.dangerContainer }]}>
@@ -164,7 +166,7 @@ export default function SearchScreen() {
 
       {!isLoading && !error && results.length === 0 && products.length === 0 && query.length >= 2 && (
         <View style={styles.emptyContainer}>
-          <Text style={[styles.emptyText, { color: colors.textSecondary }]}>No se encontraron resultados</Text>
+          <Text style={[styles.emptyText, { color: colors.textSecondary }]}>{t('search.noResults')}</Text>
         </View>
       )}
 
@@ -176,7 +178,7 @@ export default function SearchScreen() {
           <>
             {products.length > 0 && (
               <View style={styles.section}>
-                <Text style={[styles.sectionTitle, { color: colors.text }]}>Parafarmacia</Text>
+                <Text style={[styles.sectionTitle, { color: colors.text }]}>{t('search.parapharmacy')}</Text>
                 {products.map((product) => (
                   <TouchableOpacity
                     key={`${product.source}-${product.id || product._id}`}
@@ -187,7 +189,7 @@ export default function SearchScreen() {
                     <View style={styles.productInfo}>
                       <View style={styles.badges}>
                         <View style={[styles.sourceBadge, { backgroundColor: '#16a34a' }]}>
-                          <Text style={styles.badgeText}>Parafarmacia</Text>
+                          <Text style={styles.badgeText}>{t('search.parapharmacy')}</Text>
                         </View>
                       </View>
                       <Text style={[styles.productName, { color: colors.text }]} numberOfLines={1}>{product.name}</Text>