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.

pnpm-workspace.yaml

@@ -1,4 +1,5 @@
 ignoredBuiltDependencies:
+  - esbuild
   - sharp
 
 onlyBuiltDependencies: