Centre the login buttons in the space below the fields

The button row now takes the room left under the username and password
and centres in it, so Save sits in the middle of that space rather than
hugging the last field.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/app/camera-login.tsx

@@ -104,7 +104,8 @@ export function CredentialsForm({
           {error === save.error && error.kind === "auth" ? "Camera rejected that login." : error.message}
         </div>
       )}
-      <div className="flex gap-2">
+      {/* The buttons take the room left below the fields, centred in it. */}
+      <div className="flex flex-1 flex-wrap items-center justify-center gap-2">
         <button
           type="submit"
           disabled={busy}