fix(app-store): handle orphaned stack directories on template deploy (#530)

* fix(app-store): handle orphaned stack directories on template deploy

When a stack deployed via the App Store is later removed through Docker
Desktop or the CLI (instead of through Sencho), its directory remains on
disk without a compose file. The deploy endpoint previously rejected any
re-deploy with a 409 if the directory existed, even if empty.

Now the endpoint checks for a compose file before rejecting. If the
directory exists but contains no compose file, it is treated as an
orphaned remnant: cleaned up automatically and the deploy proceeds.

Also makes FileSystemService.hasComposeFile public so the deploy
endpoint can reuse it instead of duplicating the compose file check.

* docs(app-store): document orphaned stack directory cleanup behavior

backend/src/services/FileSystemService.ts

@@ -34,7 +34,7 @@ export class FileSystemService {
     return new FileSystemService(nodeId);
   }
 
-  private async hasComposeFile(dir: string): Promise<boolean> {
+  async hasComposeFile(dir: string): Promise<boolean> {
     const composeFiles = ['compose.yaml', 'compose.yml', 'docker-compose.yaml', 'docker-compose.yml'];
     for (const file of composeFiles) {
       try {