feat: replace AtlasChat with assistant-ui Thread component

- Install @assistant-ui/react and @assistant-ui/react-markdown
- components/assistant-ui/thread.tsx — full Thread UI with primitives
- components/assistant-ui/markdown-text.tsx — GFM markdown renderer
- AtlasChat.tsx — useLocalRuntime adapter calling existing atlas-chat API

Gives proper markdown rendering, branch switching, copy/retry actions,
cancel button during streaming, and a polished thread experience.

Made-with: Cursor

package.json

@@ -19,13 +19,15 @@
     "prisma:generate": "prisma generate"
   },
   "dependencies": {
+    "@assistant-ui/react": "^0.12.14",
+    "@assistant-ui/react-markdown": "^0.12.5",
     "@auth/core": "^0.34.3",
-    "@next-auth/prisma-adapter": "^1.0.7",
-    "@prisma/client": "^5.22.0",
     "@google-cloud/vertexai": "^1.10.0",
     "@google/genai": "^1.30.0",
     "@google/generative-ai": "^0.24.1",
     "@modelcontextprotocol/sdk": "^1.22.0",
+    "@next-auth/prisma-adapter": "^1.0.7",
+    "@prisma/client": "^5.22.0",
     "@radix-ui/react-alert-dialog": "^1.1.15",
     "@radix-ui/react-dialog": "^1.1.15",
     "@radix-ui/react-dropdown-menu": "^2.1.16",
@@ -48,14 +50,17 @@
     "next-auth": "^4.24.13",
     "next-themes": "^0.4.6",
     "pg": "^8.16.3",
+    "radix-ui": "^1.4.3",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
+    "remark-gfm": "^4.0.1",
     "sonner": "^2.0.7",
     "tailwind-merge": "^3.4.0",
     "tsx": "^4.20.6",
     "uuid": "^13.0.0",
     "v0-sdk": "^0.14.0",
-    "zod": "^3.23.8"
+    "zod": "^3.23.8",
+    "zustand": "^5.0.11"
   },
   "devDependencies": {
     "@tailwindcss/postcss": "^4",