fix(admin): a branding save no longer pins every default over tokens.css

The form posted every field, pre-filled with the Cafe Deck defaults, and
the save stored each one in platform_theme, where stored values beat
tokens.css: one save would have turned the app indigo again. It now sends
only the fields that changed, and its defaults are the Night Arcade
values. Pawn uploads go to the ludo_token_* slots the board now reads.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>