feat: add frontend auth pages, app layout, and Supabase integration (Phase 1E)

- Auth pages: login (password + magic link), register (with firm name), callback
- Supabase client setup: browser client, server client, middleware for session refresh
- App layout: sidebar (Dashboard, Akten, Fristen, Termine, AI Analyse, Einstellungen),
  header with user info and tenant switcher
- Shared: API client with auth headers, TypeScript types matching Go models,
  QueryClientProvider + Toaster providers
- Dependencies: @supabase/supabase-js, @supabase/ssr, @tanstack/react-query,
  lucide-react, date-fns, sonner

frontend/src/app/(app)/page.tsx

@@ -0,0 +1,10 @@
+export default function DashboardPage() {
+  return (
+    <div>
+      <h1 className="text-lg font-semibold text-neutral-900">Dashboard</h1>
+      <p className="mt-1 text-sm text-neutral-500">
+        Willkommen bei KanzlAI
+      </p>
+    </div>
+  );
+}