feat(ollama): add availability check before model operations

Add `ensureOllamaAvailable` function to lib/ollama.ts for checking and waiting for Ollama server health with configurable timeout.

Integrate the check in GET /api/ollama/models route with 3s timeout to prevent UI blocking, and in POST /api/ollama/pull route with 20s timeout to ensure availability before pulling models.

Update ChatContainer to always render ModelDropdown for better UX when no model is selected.

package.json

@@ -3,7 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "scripts": {
-    "predev": "tsx scripts/init-db.ts",
+    "predev": "powershell -ExecutionPolicy Bypass -File scripts/ensure-ollama.ps1 && tsx scripts/init-db.ts",
     "prebuild": "tsx scripts/init-db.ts",
     "dev": "next dev",
     "build": "next build",