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/signup/page.tsx

@@ -77,7 +77,7 @@ export default function SignupPage() {
 
   if (submitted) {
     return (
-      <div className="card">
+      <div className="card form-card">
         <h1>Check your email</h1>
         <p>
           We sent a confirmation link to <strong>{email}</strong>. Click the
@@ -106,7 +106,7 @@ export default function SignupPage() {
   }
 
   return (
-    <div className="card">
+    <div className="card form-card">
       <h1>Sign up</h1>
       <form onSubmit={onSubmit}>
         <label htmlFor="email">Email</label>