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/app/components/Providers.tsx

@@ -0,0 +1,17 @@
+"use client";
+
+import { ReactNode } from "react";
+import { LanguageProvider } from "../hooks/useLanguage";
+import { IntlProvider } from "./IntlProvider";
+
+interface ProvidersProps {
+  children: ReactNode;
+}
+
+export function Providers({ children }: ProvidersProps) {
+  return (
+    <LanguageProvider>
+      <IntlProvider>{children}</IntlProvider>
+    </LanguageProvider>
+  );
+}