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>

index.html

@@ -30,7 +30,7 @@
     <link rel="preconnect" href="https://fonts.googleapis.com" crossorigin />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />
     <link
-      href="https://fonts.googleapis.com/css2?family=JetBrains+Mono:ital,wght@0,400;0,600;0,700;1,400&family=VT323&display=swap"
+      href="https://fonts.googleapis.com/css2?family=Fira+Code:wght@400;500;600&family=JetBrains+Mono:ital,wght@0,400;0,600;0,700;1,400&family=VT323&display=swap"
       rel="stylesheet"
     />
     <link id="favicon" rel="shortcut icon" href="./public/favicon.ico" />