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/_layout.tsx

@@ -8,6 +8,7 @@ import { SafeAreaProvider } from 'react-native-safe-area-context';
 import { useAuthStore } from '../store/authStore';
 import { registerForPushNotifications, addNotificationListener, addNotificationResponseListener } from '../services/notifications';
 import { ThemeProvider, useThemeContext } from '../components/ThemeProvider';
+import { LanguageProvider, useTranslation } from '../src/i18n';
 import { initFaro } from '../services/faro';
 
 // Boot Faro RUM once, as early as possible.
@@ -21,6 +22,7 @@ const bgDark = require('../assets/bg_dark.png');
 function RootLayoutInner() {
   const { checkAuth } = useAuthStore();
   const { colors, isDark } = useThemeContext();
+  const { t } = useTranslation();
   const notificationListener = useRef<ReturnType<typeof addNotificationListener>>();
   const responseListener = useRef<ReturnType<typeof addNotificationResponseListener>>();
 
@@ -56,30 +58,30 @@ function RootLayoutInner() {
         <Stack.Screen name="(tabs)" options={{ headerShown: false }} />
         <Stack.Screen
           name="medicine/[id]"
-          options={{ title: 'Medicamento' }}
+          options={{ title: t('stack.medicine') }}
         />
         <Stack.Screen
           name="pharmacy/[id]"
-          options={{ title: 'Farmacia' }}
+          options={{ title: t('stack.pharmacy') }}
         />
         <Stack.Screen
           name="scanner"
           options={{
-            title: 'Escanear',
+            title: t('stack.scanner'),
             headerShown: false,
           }}
         />
         <Stack.Screen
           name="auth/login"
           options={{
-            title: 'Iniciar Sesión',
+            title: t('stack.login'),
             headerShown: false,
           }}
         />
         <Stack.Screen
           name="auth/register"
           options={{
-            title: 'Registrarse',
+            title: t('stack.register'),
             headerShown: false,
           }}
         />
@@ -112,11 +114,13 @@ export default function RootLayout() {
     <GestureHandlerRootView style={styles.root}>
       <SafeAreaProvider>
         <QueryClientProvider client={queryClient}>
-          <ThemeProvider>
-            <ThemedBackground>
-              <RootLayoutInner />
-            </ThemedBackground>
-          </ThemeProvider>
+          <LanguageProvider>
+            <ThemeProvider>
+              <ThemedBackground>
+                <RootLayoutInner />
+              </ThemedBackground>
+            </ThemeProvider>
+          </LanguageProvider>
         </QueryClientProvider>
       </SafeAreaProvider>
     </GestureHandlerRootView>