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/invites/page.tsx

@@ -4,6 +4,7 @@ import { useState } from "react";
 import { invitesApi } from "../api";
 import { PageLayout } from "../components/PageLayout";
 import { StatusBadge } from "../components/StatusBadge";
+import { EmptyState } from "../components/EmptyState";
 import { useRequireAuth } from "../hooks/useRequireAuth";
 import { useAsyncData } from "../hooks/useAsyncData";
 import { components } from "../generated/api";
@@ -49,9 +50,9 @@ export default function InvitesPage() {
   };
 
   const { READY, SPENT, REVOKED } = constants.inviteStatuses;
-  const readyInvites = invites.filter((i) => i.status === READY);
-  const spentInvites = invites.filter((i) => i.status === SPENT);
-  const revokedInvites = invites.filter((i) => i.status === REVOKED);
+  const readyInvites = (invites ?? []).filter((i) => i.status === READY);
+  const spentInvites = (invites ?? []).filter((i) => i.status === SPENT);
+  const revokedInvites = (invites ?? []).filter((i) => i.status === REVOKED);
 
   return (
     <PageLayout
@@ -67,7 +68,7 @@ export default function InvitesPage() {
           </p>
         </div>
 
-        {invites.length === 0 ? (
+        {(invites?.length ?? 0) === 0 ? (
           <EmptyState
             message="You don't have any invites yet."
             hint="Contact an admin if you need invite codes to share."