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/auth/login.tsx

@@ -22,6 +22,7 @@ import {
   saveBiometricCredentials,
   getBiometricUsername,
 } from '../../services/biometrics';
+import { useTranslation } from '../../src/i18n';
 
 type Tab = 'login' | 'register';
 
@@ -29,6 +30,7 @@ export default function LoginScreen() {
   const router = useRouter();
   const { login } = useAuthStore();
   const { colors } = useThemeContext();
+  const { t } = useTranslation();
   const [tab, setTab] = useState<Tab>('login');
   const [username, setUsername] = useState('');
   const [password, setPassword] = useState('');
@@ -64,17 +66,17 @@ export default function LoginScreen() {
 
   const handleSubmit = async () => {
     if (!username.trim() || !password) {
-      Alert.alert('Error', 'Por favor completa todos los campos');
+      Alert.alert(t('login.alert.error'), t('login.alert.fillFields'));
       return;
     }
 
     if (isRegister) {
       if (password.length < 8) {
-        Alert.alert('Error', 'La contraseña debe tener al menos 8 caracteres');
+        Alert.alert(t('login.alert.error'), t('login.alert.passwordLength'));
         return;
       }
       if (password !== confirmPassword) {
-        Alert.alert('Error', 'Las contraseñas no coinciden');
+        Alert.alert(t('login.alert.error'), t('login.alert.passwordMismatch'));
         return;
       }
     }
@@ -83,8 +85,8 @@ export default function LoginScreen() {
     try {
       if (isRegister) {
         await register(username.trim(), password);
-        Alert.alert('Éxito', 'Cuenta creada correctamente', [
-          { text: 'OK', onPress: () => setTab('login') },
+        Alert.alert(t('login.alert.success'), t('login.alert.accountCreated'), [
+          { text: t('login.alert.ok'), onPress: () => setTab('login') },
         ]);
       } else {
         await login(username.trim(), password);
@@ -95,8 +97,8 @@ export default function LoginScreen() {
       }
     } catch (error) {
       Alert.alert(
-        'Error',
-        isRegister ? 'No se pudo crear la cuenta' : 'Credenciales incorrectas'
+        t('login.alert.error'),
+        isRegister ? t('login.alert.createError') : t('login.alert.wrongCredentials')
       );
     } finally {
       setIsLoading(false);
@@ -105,7 +107,7 @@ export default function LoginScreen() {
 
   const handleBiometricLogin = async () => {
     if (!biometricUsername) {
-      Alert.alert('Error', 'No hay credenciales biométricas guardadas');
+      Alert.alert(t('login.alert.error'), t('login.alert.noBiometrics'));
       return;
     }
 
@@ -116,10 +118,10 @@ export default function LoginScreen() {
         await login(biometricUsername, '');
         router.replace('/(tabs)');
       } else {
-        Alert.alert('Error', 'Autenticación biométrica fallida');
+        Alert.alert(t('login.alert.error'), t('login.alert.biometricFailed'));
       }
     } catch (error) {
-      Alert.alert('Error', 'Error en la autenticación biométrica');
+      Alert.alert(t('login.alert.error'), t('login.alert.biometricError'));
     } finally {
       setIsLoading(false);
     }
@@ -158,7 +160,7 @@ export default function LoginScreen() {
                 !isRegister && styles.tabTextActive,
               ]}
             >
-              Iniciar Sesión
+              {t('login.tab.login')}
             </Text>
           </TouchableOpacity>
           <TouchableOpacity
@@ -176,7 +178,7 @@ export default function LoginScreen() {
                 isRegister && styles.tabTextActive,
               ]}
             >
-              Crear Cuenta
+              {t('login.tab.register')}
             </Text>
           </TouchableOpacity>
         </View>
@@ -185,12 +187,12 @@ export default function LoginScreen() {
         <View style={[styles.card, { backgroundColor: colors.card }, shadows.card]}>
           {/* Header */}
           <Text style={[styles.title, { color: colors.text }]}>
-            {isRegister ? 'Crea tu cuenta' : 'Bienvenido de nuevo'}
+            {isRegister ? t('login.title.register') : t('login.title.login')}
           </Text>
           <Text style={[styles.subtitle, { color: colors.textSecondary }]}>
             {isRegister
-              ? 'Guarda tu dirección y recibe notificaciones cuando lleguen medicamentos.'
-              : 'Inicia sesión para gestionar tu perfil y notificaciones.'}
+              ? t('login.subtitle.register')
+              : t('login.subtitle.login')}
           </Text>
 
           {/* Username */}
@@ -200,7 +202,7 @@ export default function LoginScreen() {
               style={[styles.input, { color: colors.text }]}
               value={username}
               onChangeText={setUsername}
-              placeholder="Usuario"
+              placeholder={t('login.username')}
               placeholderTextColor={colors.textSecondary}
               autoCapitalize="none"
               autoCorrect={false}
@@ -215,7 +217,7 @@ export default function LoginScreen() {
               style={[styles.input, { color: colors.text }]}
               value={password}
               onChangeText={setPassword}
-              placeholder="Contraseña"
+              placeholder={t('login.password')}
               placeholderTextColor={colors.textSecondary}
               secureTextEntry
               autoComplete={isRegister ? 'new-password' : 'current-password'}
@@ -230,7 +232,7 @@ export default function LoginScreen() {
                 style={[styles.input, { color: colors.text }]}
                 value={confirmPassword}
                 onChangeText={setConfirmPassword}
-                placeholder="Confirmar contraseña"
+                placeholder={t('login.confirmPassword')}
                 placeholderTextColor={colors.textSecondary}
                 secureTextEntry
                 autoComplete="new-password"
@@ -242,10 +244,10 @@ export default function LoginScreen() {
           {isRegister && (
             <View style={styles.hints}>
               <Text style={[styles.hint, { color: colors.textSecondary }]}>
-                <Ionicons name="information-circle-outline" size={14} color={colors.textSecondary} /> 3-32 caracteres para el usuario
+                <Ionicons name="information-circle-outline" size={14} color={colors.textSecondary} /> {t('login.hint.username')}
               </Text>
               <Text style={[styles.hint, { color: colors.textSecondary }]}>
-                <Ionicons name="information-circle-outline" size={14} color={colors.textSecondary} /> Mínimo 8 caracteres para la contraseña
+                <Ionicons name="information-circle-outline" size={14} color={colors.textSecondary} /> {t('login.hint.password')}
               </Text>
             </View>
           )}
@@ -260,7 +262,7 @@ export default function LoginScreen() {
               <Ionicons name="hourglass" size={20} color={colors.onPrimaryContainer} />
             ) : (
               <Text style={[styles.buttonText, { color: colors.onPrimaryContainer }]}>
-                {isRegister ? 'Crear Cuenta' : 'Iniciar Sesión'}
+                {isRegister ? t('login.tab.register') : t('login.tab.login')}
               </Text>
             )}
           </TouchableOpacity>
@@ -273,7 +275,7 @@ export default function LoginScreen() {
               disabled={isLoading}
             >
               <Ionicons name="finger-print" size={22} color={colors.primary} />
-              <Text style={[styles.biometricText, { color: colors.primary }]}>Iniciar con biometría</Text>
+              <Text style={[styles.biometricText, { color: colors.primary }]}>{t('login.biometric')}</Text>
             </TouchableOpacity>
           )}
         </View>
@@ -284,7 +286,7 @@ export default function LoginScreen() {
           onPress={() => setTab(isRegister ? 'login' : 'register')}
         >
           <Text style={[styles.linkText, { color: colors.primary }]}>
-            {isRegister ? '¿Ya tienes cuenta? Inicia sesión' : '¿No tienes cuenta? Regístrate'}
+            {isRegister ? t('login.link.toLogin') : t('login.link.toRegister')}
           </Text>
         </TouchableOpacity>
       </View>