feat: branding admin panel + asset registry

Admin panel (/admin/branding.php):
- Hardcoded login (admin / El3ab@Brand2024)
- Color editor for all 13 platform colors with live preview
- Game color editor (8 game-specific colors)
- Chess board color editor (5 board theme colors)
- Animation/motion settings (speeds, scale, radius)
- 19 asset upload slots with size hints:
  - Logo (200×60), Icon (64×64), Splash (1080×1920)
  - Coin/Gem/XP icons (24×24)
  - Game icons (64×64 each)
  - Win/Loss/Draw icons (128×128)
  - Medals (32×32), Avatar default (128×128)
- Each upload shows expected dimensions in Arabic
- Changes save to theme.json and appear as platform defaults
- Does NOT override user skins/cosmetics

Asset Registry (ASSET_REGISTRY.md):
- 44 emojis cataloged with file locations
- 71 colors mapped
- 9 gradients
- 17 animations
- 5 board visual configs
- 1 piece renderer (vector paths)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>