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/app/(main)/layout.tsx

@@ -10,11 +10,11 @@ export default async function MainLayout({
   const session = await auth();
 
   return (
-    <div className="flex min-h-screen flex-col">
+    <div className="flex h-screen flex-col">
       <HeaderWrapper user={session?.user} />
-      <div className="flex flex-1">
-        <Sidebar className="shrink-0 border-r" user={session?.user} />
-        <main className="flex-1">
+      <div className="flex flex-1 overflow-hidden">
+        <Sidebar className="shrink-0 border-r overflow-y-auto" user={session?.user} />
+        <main className="flex-1 overflow-y-auto">
           {children}
         </main>
       </div>