feat: add per-model unload endpoint and UI button

- POST /models/{model_name}/unload — unloads a specific model from
  memory without deleting from disk, supports all engine types
- Frontend: Unload button in model detail dialog when model is loaded
- Delete button remains disabled while loaded (unload first)

app/src/lib/api/client.ts

@@ -337,6 +337,12 @@ class ApiClient {
     });
   }
 
+  async unloadModel(modelName: string): Promise<{ message: string }> {
+    return this.request<{ message: string }>(`/models/${modelName}/unload`, {
+      method: 'POST',
+    });
+  }
+
   async cancelDownload(modelName: string): Promise<{ message: string }> {
     return this.request<{ message: string }>('/models/download/cancel', {
       method: 'POST',