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/StockBadge.tsx

@@ -2,6 +2,7 @@ import React from 'react';
 import { View, Text, StyleSheet } from 'react-native';
 import { useThemeContext } from './ThemeProvider';
 import { borderRadius, spacing } from '../constants/theme';
+import { useTranslation } from '../src/i18n';
 
 interface StockBadgeProps {
   stock: number;
@@ -9,6 +10,7 @@ interface StockBadgeProps {
 
 export function StockBadge({ stock }: StockBadgeProps) {
   const { colors, isDark } = useThemeContext();
+  const { t } = useTranslation();
 
   const getBadgeColors = () => {
     if (stock === 0) {
@@ -25,7 +27,7 @@ export function StockBadge({ stock }: StockBadgeProps) {
   return (
     <View style={[styles.badge, { backgroundColor: badgeColors.bg }]}>
       <Text style={[styles.text, { color: badgeColors.text }]}>
-        {stock === 0 ? 'Sin stock' : stock < 5 ? `Bajo (${stock})` : `Disponible (${stock})`}
+        {stock === 0 ? t('stockBadge.outOfStock') : stock < 5 ? `${t('stockBadge.lowStock')} (${stock})` : `${t('stockBadge.inStock')} (${stock})`}
       </Text>
     </View>
   );