Extend Settings > Login (beyond the existing style toggle + logo) toward
Loginfy parity: background image or color, form-panel background, button/
focus-accent color, and a custom login CSS field — all dependent on
"Style login page" and all falling back to the original fixed dark theme
when unset. LoginStyle.php sanitizes colors via sanitize_hex_color(),
esc_url()s the background image, and appends custom CSS (</style> stripped).
Verified on wp-login.php.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
