fix(stacks): resolve permission denied error on stack deletion (#261)

* fix(stacks): resolve permission denied error when deleting stacks with root-owned files

When Docker Compose creates files as root inside a stack directory, the
non-root Sencho process cannot remove them. This adds a Docker-based
fallback: if fsPromises.rm fails with EACCES/EPERM, Sencho spawns a
short-lived Alpine container to clean up the root-owned files.

Also enhances docker compose down with --volumes --remove-orphans to let
Docker clean up its own resources before filesystem deletion.

* docs: clarify that pre-existing root-owned stacks can be deleted

* fix(stacks): include Docker stderr in fallback deletion error message

Fixes CI lint failure: 'stderr' was assigned but never read in
forceDeleteViaDocker(). Now surfaces Docker stderr output in the error
message when the fallback cleanup fails.

backend/src/services/ComposeService.ts

@@ -378,7 +378,7 @@ export class ComposeService {
   public async downStack(stackName: string): Promise<void> {
     const stackPath = path.join(this.baseDir, stackName);
     try {
-      await this.execute('docker', ['compose', 'down'], stackPath, undefined, false);
+      await this.execute('docker', ['compose', 'down', '--volumes', '--remove-orphans'], stackPath, undefined, false);
     } catch (error) {
       console.warn(`[Teardown] Docker down failed or nothing to clean up for ${stackName}`);
     }