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/components/MedicineCard.tsx

@@ -6,6 +6,7 @@ import { useThemeContext } from './ThemeProvider';
 import { spacing, borderRadius } from '../constants/theme';
 import { StockBadge } from './StockBadge';
 import { Medicine } from '../types';
+import { useTranslation } from '../src/i18n';
 
 const TABLET_MIN_WIDTH = 768;
 
@@ -18,6 +19,7 @@ export function MedicineCard({ medicine }: MedicineCardProps) {
   const { width } = useWindowDimensions();
   const isTablet = width >= TABLET_MIN_WIDTH;
   const { colors } = useThemeContext();
+  const { t } = useTranslation();
 
   const handlePress = () => {
     router.push(`/medicine/${medicine.nregistro}`);
@@ -44,7 +46,7 @@ export function MedicineCard({ medicine }: MedicineCardProps) {
         <View style={styles.priceContainer}>
           <Ionicons name="pricetag" size={14} color={colors.textSecondary} />
           <Text style={[styles.price, isTablet && styles.priceTablet, { color: colors.primary }]}>
-            {medicine.precio != null ? `${medicine.precio.toFixed(2)} €` : 'Sin precio'}
+            {medicine.precio != null ? `${medicine.precio.toFixed(2)} €` : t('medicineCard.noPrice')}
           </Text>
         </View>