feat(auth): restyle the auth pages on the ruled frame

[CodeWithEugene]

This commit was pushed to the #54 branch after that PR had already been merged, so it never reached main. This PR lands it.

What changes

The half photo, half form split stays as it is.

• Form side:
  • The form now sits in the site's ruled frame: a soft tint at the top and hairline rules down both sides.
  • A new top bar puts the logo on the left, with Home and the theme switch on the right.
  • Form cards get a hairline border instead of a heavy shadow, and headings use the lighter display weight.
  • The "Welcome Back" heading on sign-in is now left-aligned.
• Photo side:
  • Same photo and copy.
  • The label drops its uppercase tracking and the heading uses the lighter weight.
  • The 50% card becomes a glass card with a tick-line stat, replacing the hard offset shadow.

This applies to every page under app/(auth). No copy changes.

Google and GitHub sign-in

Untouched. OAuthButtons still renders whenever GOOGLE_CLIENT_ID / GITHUB_CLIENT_ID are set, as they are in production. I checked with placeholder client IDs locally: both buttons show on sign-in and sign-up in the new layout, in light and dark mode.

Checks

• tsc --noEmit and next lint
• Commit message passes commitlint
• Sign-in, sign-up, forgot password, reset password and verify email have no horizontal overflow at 375px