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)/_layout.tsx

@@ -3,6 +3,7 @@ import { Ionicons } from '@expo/vector-icons';
 import { View, StyleSheet, useWindowDimensions } from 'react-native';
 import { useSafeAreaInsets } from 'react-native-safe-area-context';
 import { useThemeContext } from '../../components/ThemeProvider';
+import { useTranslation } from '../../src/i18n';
 import { shadows } from '../../constants/theme';
 
 const TABLET_MIN_WIDTH = 768;
@@ -25,6 +26,7 @@ export default function TabLayout() {
   const { width } = useWindowDimensions();
   const isTablet = width >= TABLET_MIN_WIDTH;
   const { colors } = useThemeContext();
+  const { t } = useTranslation();
   // Use safe area insets if available, otherwise use standard Android nav bar height
   const bottomPadding = insets.bottom > 0 ? insets.bottom : ANDROID_NAV_BAR_HEIGHT;
 
@@ -62,7 +64,7 @@ export default function TabLayout() {
       <Tabs.Screen
         name="index"
         options={{
-          title: 'Inicio',
+          title: t('nav.home'),
           tabBarIcon: ({ color, size }) => (
             <Ionicons name="home" size={size} color={color} />
           ),
@@ -71,7 +73,7 @@ export default function TabLayout() {
       <Tabs.Screen
         name="search"
         options={{
-          title: 'Buscar',
+          title: t('nav.search'),
           tabBarIcon: ({ color, size }) => (
             <Ionicons name="search" size={size} color={color} />
           ),
@@ -80,7 +82,7 @@ export default function TabLayout() {
       <Tabs.Screen
         name="scan"
         options={{
-          title: 'Escanear',
+          title: t('nav.scan'),
           tabBarIcon: ({ color, size }) => <ScanIcon color={color} size={size} />,
           tabBarLabel: () => null,
         }}
@@ -88,7 +90,7 @@ export default function TabLayout() {
       <Tabs.Screen
         name="alerts"
         options={{
-          title: 'Avisos',
+          title: t('nav.alerts'),
           tabBarIcon: ({ color, size }) => (
             <Ionicons name="notifications" size={size} color={color} />
           ),
@@ -103,7 +105,7 @@ export default function TabLayout() {
       <Tabs.Screen
         name="profile"
         options={{
-          title: 'Perfil',
+          title: t('nav.profile'),
           tabBarIcon: ({ color, size }) => (
             <Ionicons name="person" size={size} color={color} />
           ),