fix(stacks): default Empty template ships ports block commented out (#1189)

The Empty branch of the Create Stack flow wrote a compose.yaml whose
first service bound the host's port 8080 by default. On any workstation
already running something on 8080 (traefik, caddy, librespeed, another
nginx, etc.) the very first deploy failed at the docker compose
networking step with "Bind for 0.0.0.0:8080 failed: port is already
allocated", which made the day-one experience feel broken right after
F-2 (PR #1168) tightened the dialog itself.

The boilerplate in FileSystemService.createStack now emits the ports
block commented out plus a one-line hint above it. A fresh deploy
binds no host port, so the container starts cleanly on any host; the
user uncomments the two-line block when they're ready to expose the
container. The deterministic shape (no probe-and-write, no random port,
no preflight scan) avoids the TOCTOU race that a free-port probe would
have left between template creation and the actual compose up.

Adds backend/src/__tests__/file-system-service-create-stack.test.ts
(8 cases): directory + file creation, structural YAML assertions via
yaml.parse to lock in the no-live-ports invariant, raw-text regex to
lock in the commented hint, and the already-exists rejection path.
FileSystemService.createStack had no coverage before this change.

Adds e2e/default-stack-template-no-fixed-port.spec.ts (1 case): drives
the dialog through Create, reads the resulting compose via the
in-browser apiFetch, and asserts the live + commented invariants
end-to-end.

docs/features/stack-management.mdx Empty bullet rewritten to describe
the minimal skeleton and the commented ports block instead of calling
it "blank".

Resolves: F-3 in the v1.0 audit tracker.

docs/features/stack-management.mdx

@@ -13,7 +13,7 @@ A **stack** in Sencho is a Docker Compose project: a directory inside your `COMP
 
 Click **Create Stack** in the left sidebar to open the **New stack** dialog. Pick a source from the tabs at the top:
 
-- **Empty**: start with a blank `compose.yaml` and write it from scratch.
+- **Empty**: start with a minimal `compose.yaml` skeleton. The default service uses `nginx:latest` with the host port binding commented out, so a fresh deploy never collides with an existing service on the host. Uncomment and adjust the `ports:` block when you're ready to expose the container.
 - **From Git**: clone a Git repository so its compose file becomes the stack source. Future webhook pulls keep it in sync. See [Git Sources](/features/git-sources) for the full sync flow.
 - **From Docker Run**: paste a `docker run` command and let Sencho convert it to compose YAML.