feat: add global font settings with per-project override

- Add fontFamilySans and fontFamilyMono to GlobalSettings type
- Add global font state and actions to app store
- Update getEffectiveFontSans/Mono to fall back to global settings
- Add font selectors to global Settings → Appearance
- Add "Use Global Font" checkboxes in Project Settings → Theme
- Add fonts to settings sync and migration
- Include fonts in import/export JSON

apps/ui/src/hooks/use-settings-sync.ts

@@ -33,6 +33,8 @@ const SYNC_DEBOUNCE_MS = 1000;
 // Fields to sync to server (subset of AppState that should be persisted)
 const SETTINGS_FIELDS_TO_SYNC = [
   'theme',
+  'fontFamilySans',
+  'fontFamilyMono',
   'sidebarOpen',
   'chatHistoryOpen',
   'maxConcurrency',