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/App.test.jsx

@@ -1,8 +1,13 @@
 import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
 import { render, screen, fireEvent, act } from '@testing-library/react'
+import { LanguageProvider } from './i18n'
 import HomeView from './views/HomeView.jsx'
 import SearchView from './views/SearchView.jsx'
 
+function Wrapper({ children }) {
+  return <LanguageProvider>{children}</LanguageProvider>
+}
+
 beforeEach(() => {
   vi.useFakeTimers()
 })
@@ -16,14 +21,14 @@ describe('HomeView', () => {
   it('renders two action buttons on the home screen', () => {
     const onSearch = vi.fn()
     const onScan = vi.fn()
-    render(<HomeView onSearchClick={onSearch} onScanClick={onScan} />)
+    render(<HomeView onSearchClick={onSearch} onScanClick={onScan} />, { wrapper: Wrapper })
     expect(screen.getByRole('button', { name: /buscar medicamento/i })).toBeInTheDocument()
     expect(screen.getByRole('button', { name: /escanear tsi/i })).toBeInTheDocument()
   })
 
   it('calls onSearchClick when Buscar Medicamento is clicked', async () => {
     const onSearch = vi.fn()
-    render(<HomeView onSearchClick={onSearch} onScanClick={vi.fn()} />)
+    render(<HomeView onSearchClick={onSearch} onScanClick={vi.fn()} />, { wrapper: Wrapper })
     fireEvent.click(screen.getByRole('button', { name: /buscar medicamento/i }))
     expect(onSearch).toHaveBeenCalledTimes(1)
   })
@@ -31,13 +36,13 @@ describe('HomeView', () => {
 
 describe('SearchView', () => {
   it('renders search bar with placeholder', () => {
-    render(<SearchView />)
+    render(<SearchView />, { wrapper: Wrapper })
     expect(screen.getByPlaceholderText(/escriba el nombre del medicamento/i)).toBeInTheDocument()
   })
 
   it('does not fetch for queries shorter than 2 chars', async () => {
     const fetchMock = vi.spyOn(globalThis, 'fetch')
-    render(<SearchView />)
+    render(<SearchView />, { wrapper: Wrapper })
 
     fireEvent.change(screen.getByPlaceholderText(/escriba el nombre del medicamento/i), {
       target: { value: 'a' },
@@ -59,7 +64,7 @@ describe('SearchView', () => {
       json: async () => medicines,
     })
 
-    render(<SearchView />)
+    render(<SearchView />, { wrapper: Wrapper })
 
     fireEvent.change(screen.getByPlaceholderText(/escriba el nombre del medicamento/i), {
       target: { value: 'ibu' },
@@ -81,7 +86,7 @@ describe('SearchView', () => {
       json: async () => medicines,
     })
 
-    render(<SearchView />)
+    render(<SearchView />, { wrapper: Wrapper })
     const input = screen.getByPlaceholderText(/escriba el nombre del medicamento/i)
 
     fireEvent.change(input, { target: { value: 'ibu' } })