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)