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/SearchBar.tsx

@@ -3,6 +3,7 @@ import { View, TextInput, StyleSheet, TouchableOpacity, useWindowDimensions } fr
 import { Ionicons } from '@expo/vector-icons';
 import { useThemeContext } from './ThemeProvider';
 import { spacing, borderRadius } from '../constants/theme';
+import { useTranslation } from '../src/i18n';
 
 const TABLET_MIN_WIDTH = 768;
 
@@ -14,7 +15,7 @@ interface SearchBarProps {
 }
 
 export function SearchBar({
-  placeholder = 'Buscar medicamentos...',
+  placeholder,
   onSearch,
   value,
   onChangeText
@@ -22,6 +23,7 @@ export function SearchBar({
   const { width } = useWindowDimensions();
   const isTablet = width >= TABLET_MIN_WIDTH;
   const { colors } = useThemeContext();
+  const { t } = useTranslation();
   const [localValue, setLocalValue] = useState(value || '');
 
   const handleChange = (text: string) => {
@@ -44,7 +46,7 @@ export function SearchBar({
       <Ionicons name="search" size={20} color={colors.textSecondary} style={styles.icon} />
       <TextInput
         style={[styles.input, isTablet && styles.inputTablet, { color: colors.text }]}
-        placeholder={placeholder}
+        placeholder={placeholder ?? t('search.placeholder')}
         placeholderTextColor={colors.textSecondary}
         value={localValue}
         onChangeText={handleChange}