refactor: clean up dead files, restructure nav, align QuickStartPage to monochrome

- Delete 6 unused files: ThemeToggle, themeStore, AppLayout-original,
  QuickStartPage-Enhanced, UpgradePrompt, SettingsPage
- Restructure Account/Settings navigation: merge Settings into Account page,
  make AccountSettingsPage the /account index, remove orphaned /account-settings route
- Remove "Settings" nav item (consolidated under Account)
- Add export preferences and team categories link to AccountSettingsPage
- Align QuickStartPage to monochrome design system: replace cyan/blue/violet
  accent colors with white opacity variants
- Replace non-functional search button with search spinner indicator

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

frontend/src/components/layout/AppLayout.tsx

@@ -50,7 +50,6 @@ export function AppLayout() {
     { path: '/my-trees', label: 'My Trees' },
     { path: '/sessions', label: 'Sessions' },
     { path: '/account', label: 'Account' },
-    { path: '/settings', label: 'Settings' },
     ...(isSuperAdmin ? [{ path: '/admin', label: 'Admin Panel' }] : []),
   ]