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/IntlProvider.tsx

@@ -0,0 +1,34 @@
+"use client";
+
+import { NextIntlClientProvider } from "next-intl";
+import { ReactNode, useMemo } from "react";
+import { useLanguage } from "../hooks/useLanguage";
+
+// Import all locale messages
+import esCommon from "../../locales/es/common.json";
+import enCommon from "../../locales/en/common.json";
+import caCommon from "../../locales/ca/common.json";
+
+const messages = {
+  es: { common: esCommon },
+  en: { common: enCommon },
+  ca: { common: caCommon },
+};
+
+interface IntlProviderProps {
+  children: ReactNode;
+}
+
+export function IntlProvider({ children }: IntlProviderProps) {
+  const { locale } = useLanguage();
+
+  const localeMessages = useMemo(() => {
+    return messages[locale] || messages.es;
+  }, [locale]);
+
+  return (
+    <NextIntlClientProvider locale={locale} messages={localeMessages}>
+      {children}
+    </NextIntlClientProvider>
+  );
+}