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/store/themeStore.ts

@@ -1,53 +0,0 @@
-import { create } from 'zustand'
-import { persist } from 'zustand/middleware'
-
-type Theme = 'light' | 'dark' | 'system'
-
-interface ThemeState {
-  theme: Theme
-  resolvedTheme: 'light' | 'dark'
-  setTheme: (theme: Theme) => void
-}
-
-const getSystemTheme = (): 'light' | 'dark' => {
-  if (typeof window === 'undefined') return 'light'
-  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
-}
-
-const applyTheme = (theme: Theme): 'light' | 'dark' => {
-  const resolved = theme === 'system' ? getSystemTheme() : theme
-  const root = document.documentElement
-
-  if (resolved === 'dark') {
-    root.classList.add('dark')
-  } else {
-    root.classList.remove('dark')
-  }
-
-  return resolved
-}
-
-export const useThemeStore = create<ThemeState>()(
-  persist(
-    (set) => ({
-      theme: 'system',
-      resolvedTheme: getSystemTheme(),
-
-      setTheme: (theme: Theme) => {
-        const resolvedTheme = applyTheme(theme)
-        set({ theme, resolvedTheme })
-      },
-    }),
-    {
-      name: 'theme-storage',
-      onRehydrateStorage: () => (state) => {
-        // Apply theme on initial load after rehydration
-        if (state) {
-          applyTheme(state.theme)
-        }
-      },
-    }
-  )
-)
-
-export default useThemeStore