feat: P5-21 mention color, P5-22 font selector, P5-27 notification presets; update docs

- P5-21: Custom @mention highlight color picker in Settings → Appearance.
  CSS vars with luminance-computed text color; resets cleanly to theme default.
- P5-22: Font selector (System, Inter, JetBrains Mono, Fira Code) in
  Settings → Appearance. Fira Code added to Google Fonts preload.
- P5-27: Gaming/Work/Sleep preset buttons at top of Settings → Notifications.
  Each atomically applies a group of notification settings.
- AppearanceEffects component in App.tsx applies CSS vars on settings change.
- LOTUS_BUGS.md: mark presence + manifest icon bugs as resolved.
- LOTUS_TODO.md: mark P3-6, P5-21, P5-22, P5-27 as [x].
- LOTUS_FEATURES.md: document all four completed features.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/state/settings.ts

@@ -125,6 +125,9 @@ export interface Settings {
   pauseAnimations: boolean;
 
   composerToolbarButtons: ComposerToolbarSettings;
+
+  mentionHighlightColor: string;
+  fontFamily: 'system' | 'inter' | 'jetbrains-mono' | 'fira-code';
 }
 
 const defaultSettings: Settings = {
@@ -192,6 +195,9 @@ const defaultSettings: Settings = {
   pauseAnimations: false,
 
   composerToolbarButtons: DEFAULT_COMPOSER_TOOLBAR,
+
+  mentionHighlightColor: '',
+  fontFamily: 'inter',
 };
 
 export const getSettings = (): Settings => {