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/common/UpgradePrompt.tsx

@@ -1,32 +0,0 @@
-import { cn } from '@/lib/utils'
-import { useSubscription } from '@/hooks/useSubscription'
-
-interface UpgradePromptProps {
-  feature: string  // e.g., "create more trees", "start more sessions"
-  className?: string
-}
-
-export function UpgradePrompt({ feature, className }: UpgradePromptProps) {
-  const { plan } = useSubscription()
-
-  return (
-    <div className={cn(
-      'glass-card rounded-2xl border border-white/[0.06] p-4',
-      className
-    )}>
-      <h3 className="font-semibold text-white">Plan Limit Reached</h3>
-      <p className="mt-1 text-sm text-white/70">
-        Your {plan} plan doesn't allow you to {feature}. Upgrade your plan to continue.
-      </p>
-      <button
-        className={cn(
-          'mt-3 rounded-xl bg-white px-4 py-2 text-sm font-medium text-black',
-          'hover:bg-white/90'
-        )}
-        onClick={() => window.location.href = '/account'}
-      >
-        View Plans
-      </button>
-    </div>
-  )
-}