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/package.json

@@ -9,9 +9,15 @@
     "lint": "eslint"
   },
   "dependencies": {
+    "@supabase/ssr": "^0.9.0",
+    "@supabase/supabase-js": "^2.100.0",
+    "@tanstack/react-query": "^5.95.2",
+    "date-fns": "^4.1.0",
+    "lucide-react": "^1.6.0",
+    "next": "15.5.14",
     "react": "19.1.0",
     "react-dom": "19.1.0",
-    "next": "15.5.14"
+    "sonner": "^2.0.7"
   },
   "devDependencies": {
     "typescript": "^5",