Phase 1: Infrastructure setup - Install next-intl and create basic i18n structure

- Install next-intl package
- Create LanguageProvider hook with localStorage persistence
- Create IntlProvider component for next-intl integration
- Create Providers wrapper component
- Update layout.tsx to include providers and set default lang to 'es'
- Create initial translation files (common.json) for es, en, ca
- Fix pre-existing TypeScript errors in various pages

All tests passing, build successful.

frontend/package.json

@@ -19,6 +19,7 @@
   "dependencies": {
     "bech32": "^2.0.0",
     "next": "15.1.2",
+    "next-intl": "^4.6.1",
     "react": "19.0.0",
     "react-dom": "19.0.0"
   },