fix: sidebar section header reflects type icon, color, and label

- Add GearSix icon ('gear-six') to icon registry — was missing, causing
  Config type to show FileText fallback instead of its configured icon
- Add 'gray' to ACCENT_COLORS palette with CSS variables — was missing,
  causing Config type color to fall back to muted foreground
- Extract sidebar section logic to utils/sidebarSections.ts for testability
- Add Config type + instance to mock entries for browser dev mode
- Add tests: icon resolution, gray color, sidebar section builder

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

src/index.css

@@ -82,6 +82,8 @@
   --accent-teal-light: rgba(49, 151, 149, 0.1);
   --accent-pink: #D53F8C;
   --accent-pink-light: rgba(213, 63, 140, 0.1);
+  --accent-gray: #718096;
+  --accent-gray-light: rgba(113, 128, 150, 0.1);
   --border-primary: #E9E9E7;
   --border-subtle: #E9E9E7;
   --border-input: #E9E9E7;