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-migration.ts

@@ -556,6 +556,8 @@ export function hydrateStoreFromSettings(settings: GlobalSettings): void {
 
   useAppStore.setState({
     theme: settings.theme as unknown as import('@/store/app-store').ThemeMode,
+    fontFamilySans: settings.fontFamilySans ?? null,
+    fontFamilyMono: settings.fontFamilyMono ?? null,
     sidebarOpen: settings.sidebarOpen ?? true,
     chatHistoryOpen: settings.chatHistoryOpen ?? false,
     maxConcurrency: settings.maxConcurrency ?? 3,

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',