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/activate/page.tsx

@@ -24,7 +24,7 @@ export default async function ActivatePage({
     .maybeSingle<{ subdomain: string }>();
 
   return (
-    <div>
+    <div className="form-card">
       <h1>Activate device</h1>
       <p className="muted">
         Enter the code shown by your Home Assistant add-on to pair it with your