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/pages/AccountSettingsPage.tsx

@@ -1,16 +1,20 @@
 import { useEffect, useState } from 'react'
-import { Building2, Users, Mail, Crown, Loader2, AlertCircle, Check, X } from 'lucide-react'
+import { Link } from 'react-router-dom'
+import { Building2, Users, Mail, Crown, Loader2, AlertCircle, Check, X, Settings, FolderTree } from 'lucide-react'
 import { accountsApi } from '@/api'
 import type { Account, AccountMember, AccountInvite } from '@/types'
 import { cn } from '@/lib/utils'
 import { usePermissions } from '@/hooks/usePermissions'
 import { useSubscription } from '@/hooks/useSubscription'
 import { useAuthStore } from '@/store/authStore'
+import { useUserPreferencesStore } from '@/store/userPreferencesStore'
 import { CheckoutButton } from '@/components/subscription/CheckoutButton'
+import { toast } from '@/lib/toast'
 
 export function AccountSettingsPage() {
   const { isAccountOwner } = usePermissions()
   const { plan, limits, usage } = useSubscription()
+  const { defaultExportFormat, setDefaultExportFormat } = useUserPreferencesStore()
   const subscription = useAuthStore((s) => s.subscription)
 
   const [account, setAccount] = useState<Account | null>(null)
@@ -442,6 +446,60 @@ export function AccountSettingsPage() {
             )}
           </div>
         )}
+
+        {/* Team Categories Link (owners only) */}
+        {isAccountOwner && (
+          <Link
+            to="/account/categories"
+            className="glass-card rounded-2xl p-4 sm:p-6 flex items-center justify-between group hover:border-white/20 transition-all"
+          >
+            <div className="flex items-center gap-3">
+              <FolderTree className="h-5 w-5 text-white/50" />
+              <div>
+                <h2 className="text-lg font-semibold text-white">Team Categories</h2>
+                <p className="text-sm text-white/40">Manage tree categories for your team</p>
+              </div>
+            </div>
+            <span className="text-white/30 group-hover:text-white transition-colors">&rarr;</span>
+          </Link>
+        )}
+
+        {/* Preferences Section */}
+        <div className="glass-card rounded-2xl p-4 sm:p-6">
+          <div className="flex items-center gap-2">
+            <Settings className="h-5 w-5 text-white/50" />
+            <h2 className="text-lg font-semibold text-white">Preferences</h2>
+          </div>
+
+          <div className="mt-4">
+            <label
+              htmlFor="export-format"
+              className="block text-sm font-medium text-white"
+            >
+              Default Export Format
+            </label>
+            <p className="text-sm text-white/40">
+              This format will be pre-selected when exporting sessions
+            </p>
+            <select
+              id="export-format"
+              value={defaultExportFormat}
+              onChange={(e) => {
+                setDefaultExportFormat(e.target.value as 'markdown' | 'text' | 'html')
+                toast.success('Preference saved')
+              }}
+              className={cn(
+                'mt-2 block w-full max-w-xs rounded-xl border border-white/10 bg-black/50 px-3 py-2',
+                'text-sm text-white',
+                'focus:border-white/30 focus:outline-none focus:ring-1 focus:ring-white/20'
+              )}
+            >
+              <option value="markdown">Markdown (.md)</option>
+              <option value="text">Plain Text (.txt)</option>
+              <option value="html">HTML (.html)</option>
+            </select>
+          </div>
+        </div>
       </div>
     </div>
   )