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)

libs/types/src/settings.ts

@@ -718,6 +718,12 @@ export interface ProjectSettings {
   /** Project theme (undefined = use global setting) */
   theme?: ThemeMode;
 
+  // Font Configuration (project-specific override)
+  /** UI/Sans font family override (undefined = use default Geist Sans) */
+  fontFamilySans?: string;
+  /** Code/Mono font family override (undefined = use default Geist Mono) */
+  fontFamilyMono?: string;
+
   // Worktree Management
   /** Project-specific worktree preference override */
   useWorktrees?: boolean;