feat: add app preview via NodePort

- Backend: getPreviewInfo() in KubernetesService auto-patches ClusterIP
  service to NodePort for direct external access
- Backend: GET /applications/:id/preview endpoint returns access URL
  with nodePort, host IP (extracted from kubeconfig), and ingress URL
- Frontend: '🌐 Preview' button on app detail page (visible when running)
  opens the deployed app in a new browser tab via NodePort URL
- Tested: service patched to NodePort 30107 successfully

backend/src/applications/applications.controller.ts

@@ -122,6 +122,16 @@ export class ApplicationsController {
     return updated;
   }
 
+  @Get(':id/preview')
+  @ApiOperation({ summary: 'Get preview URL for the deployed application' })
+  async getPreview(@Param('id') id: string, @Request() req: any) {
+    const app = await this.applicationsService.findOne(
+      id,
+      req.user.role === UserRole.ADMIN ? undefined : req.user.id,
+    );
+    return this.kubernetesService.getPreviewInfo(app);
+  }
+
   @Delete(':id')
   @ApiOperation({ summary: 'Delete an application and all its resources' })
   async delete(@Param('id') id: string, @Request() req: any) {