feat: AI provider testing page + multi-provider support + UX design spec

- Add AI Provider Testing page (/admin/ai-test) with Tags and Embeddings tests
- Add new AI providers: CustomOpenAI, DeepSeek, OpenRouter
- Add API routes for AI config, models listing, and testing endpoints
- Add UX Design Specification document for Phase 1 MVP AI
- Add PRD Phase 1 MVP AI planning document
- Update admin settings and sidebar navigation
- Fix AI factory for multi-provider support

keep-notes/components/header.tsx

@@ -25,7 +25,6 @@ import { useLabels } from '@/context/LabelContext'
 import { LabelManagementDialog } from './label-management-dialog'
 import { LabelFilter } from './label-filter'
 import { NotificationPanel } from './notification-panel'
-import { UserNav } from './user-nav'
 import { updateTheme } from '@/app/actions/profile'
 
 interface HeaderProps {
@@ -316,7 +315,6 @@ export function Header({
             </DropdownMenu>
 
             <NotificationPanel />
-            <UserNav user={user} />
           </div>
         </div>