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

libs/types/src/settings.ts

@@ -463,6 +463,12 @@ export interface GlobalSettings {
   /** Currently selected theme */
   theme: ThemeMode;
 
+  // Font Configuration
+  /** Global UI/Sans font family (undefined = use default Geist Sans) */
+  fontFamilySans?: string;
+  /** Global Code/Mono font family (undefined = use default Geist Mono) */
+  fontFamilyMono?: string;
+
   // UI State Preferences
   /** Whether sidebar is currently open */
   sidebarOpen: boolean;