feat(ui): responsive layout for mobile and wide screens

- Add explicit viewport meta (device-width) for proper mobile scaling.
- Widen the main container (720 -> 960, 1120 on >=1280px) with fluid
  clamp() padding so wide screens are used better.
- Wrap the top nav and allow it to reflow on narrow screens.
- Collapse the key/value grid to a single column and enlarge touch
  targets on small screens.
- Keep single-form pages (login/signup/activate) in a readable narrow
  column instead of stretching full width.

app/layout.tsx

@@ -1,4 +1,4 @@
-import type { Metadata } from 'next';
+import type { Metadata, Viewport } from 'next';
 import Link from 'next/link';
 import './globals.css';
 import { createSupabaseServerClient } from '@/lib/supabase/server';
@@ -8,6 +8,11 @@ export const metadata: Metadata = {
   description: 'Remote access tunnels for Home Assistant',
 };
 
+export const viewport: Viewport = {
+  width: 'device-width',
+  initialScale: 1,
+};
+
 export const dynamic = 'force-dynamic';
 
 export default async function RootLayout({