feat(ui): unify font loading and add design system documentation

Font unification:
- Add @fontsource packages for Geist Sans, Geist Mono, Playfair Display
- Create fonts.ts for centralized font imports
- Import fonts in both web (layout.tsx) and desktop (main.tsx)
- Register --font-brand in Tailwind @theme inline block
- Fix font-brand class usage (replace arbitrary value syntax)

Design system documentation:
- Add comprehensive design philosophy comments to globals.css
- Document typography choices (why Geist, why Playfair for brand)
- Document color system approach (neutral grays, semantic colors only)
- Explain component library customizations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -71,9 +71,9 @@
     "croner": "^10.0.1",
     "fast-glob": "^3.3.3",
     "grammy": "^1.39.3",
-    "mysql2": "^3.14.1",
     "json5": "^2.2.3",
     "linkedom": "^0.18.12",
+    "mysql2": "^3.14.1",
     "nestjs-pino": "^4.5.0",
     "pino": "^10.3.0",
     "pino-http": "^11.0.0",