feat: add per-project font override settings

Add font selectors that allow per-project font customization for both
sans and mono fonts, independent of theme selection. Uses system fonts.

- Add fontFamilySans and fontFamilyMono to ProjectSettings and Project types
- Create ui-font-options.ts config with system font options
- Add store actions: setProjectFontSans, setProjectFontMono, getEffectiveFontSans, getEffectiveFontMono
- Apply font CSS variables in root component
- Add font selector UI in project-theme-section (Project Settings → Theme)

apps/ui/src/components/views/settings-view/shared/types.ts

@@ -26,6 +26,8 @@ export interface Project {
   name: string;
   path: string;
   theme?: string;
+  fontFamilySans?: string;
+  fontFamilyMono?: string;
   icon?: string;
   customIconPath?: string;
 }