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)/index.tsx

@@ -3,6 +3,7 @@ import { View, Text, StyleSheet, TouchableOpacity, Image, useWindowDimensions }
 import { Ionicons } from '@expo/vector-icons';
 import { useRouter } from 'expo-router';
 import { useThemeContext } from '../../components/ThemeProvider';
+import { useTranslation } from '../../src/i18n';
 import { spacing, borderRadius, shadows } from '../../constants/theme';
 
 const TABLET_MIN_WIDTH = 768;
@@ -12,6 +13,7 @@ export default function HomeScreen() {
   const { width } = useWindowDimensions();
   const isTablet = width >= TABLET_MIN_WIDTH;
   const { colors } = useThemeContext();
+  const { t } = useTranslation();
 
   return (
     <View style={[styles.container, { backgroundColor: colors.background }]}>
@@ -23,7 +25,7 @@ export default function HomeScreen() {
         />
         <Text style={[styles.brandName, isTablet && styles.brandNameTablet, { color: colors.text }]}>FarmaClic</Text>
         <Text style={[styles.description, isTablet && styles.descriptionTablet, { color: colors.textSecondary }]}>
-          Encuentra tus medicamentos en farmacias cercanas
+          {t('home.description')}
         </Text>
       </View>
 
@@ -37,7 +39,7 @@ export default function HomeScreen() {
             <Ionicons name="search" size={isTablet ? 28 : 24} color={colors.onPrimaryContainer} />
           </View>
           <View style={styles.cardContent}>
-            <Text style={[styles.cardLabel, isTablet && styles.cardLabelTablet, { color: colors.onPrimaryContainer }]}>Buscar Medicamento</Text>
+            <Text style={[styles.cardLabel, isTablet && styles.cardLabelTablet, { color: colors.onPrimaryContainer }]}>{t('home.searchMedicine')}</Text>
             <Ionicons name="chevron-forward" size={20} color={colors.onPrimaryContainer} style={{ opacity: 0.7 }} />
           </View>
         </TouchableOpacity>
@@ -51,7 +53,7 @@ export default function HomeScreen() {
             <Ionicons name="scan" size={isTablet ? 28 : 24} color="#ffffff" />
           </View>
           <View style={styles.cardContent}>
-            <Text style={[styles.cardLabel, styles.cardLabelScan, isTablet && styles.cardLabelTablet]}>Escanear TSI</Text>
+            <Text style={[styles.cardLabel, styles.cardLabelScan, isTablet && styles.cardLabelTablet]}>{t('home.scanTSI')}</Text>
             <Ionicons name="chevron-forward" size={20} color="#ffffff" style={{ opacity: 0.7 }} />
           </View>
         </TouchableOpacity>