feat(i18n): add bilingual support (Català / Castellano)

Add lightweight i18n system using React Context + useTranslation hook.
No external dependencies. Language persisted in localStorage('ff-lang').

New files:
- src/i18n/locales/ca.js (~410 translation keys)
- src/i18n/locales/es.js (~410 translation keys)
- src/i18n/LanguageContext.jsx (Provider + t() function)
- src/i18n/useTranslation.js (hook)
- src/i18n/index.js (barrel export)

Modified 22 files:
- main.jsx: wrap App with LanguageProvider
- App.test.jsx: add LanguageProvider wrapper for tests
- 13 user components: BottomNav, HomeView, LoginModal, SearchView,
  ProfileView, AlertsView, ScannerView, PharmacyList, MedicineResults,
  ProductResults, SavedNotifications, ErrorBoundary, SearchBar
- 6 admin components: AdminView, LoginForm, PharmacyManagement,
  MedicineManagement, PharmacyMedicineLink, PharmacyProductLink

ProfileView includes language selector (globe icon + modal CA/ES)
placed next to the Theme selector.

No routes changed. No API calls affected. No navigation changes.

apps/frontend/src/components/SearchBar.jsx

@@ -1,7 +1,9 @@
 import React from 'react';
+import { useTranslation } from '../i18n';
 import './SearchBar.css';
 
 function SearchBar({ value, onChange, placeholder }) {
+  const { t } = useTranslation();
   return (
     <div className="search-bar-container">
       <div className="search-bar">
@@ -22,7 +24,7 @@ function SearchBar({ value, onChange, placeholder }) {
           <button
             className="clear-button"
             onClick={() => onChange('')}
-            aria-label="Limpiar búsqueda"
+            aria-label={t('search.clear')}
           >
             ✕
           </button>