feat: dispatch frontend PR preview environments to cloud (#9715)

## Summary

Add support for deploying full ephemeral preview environments from
frontend PRs. This is the frontend-side half — it sends `pr_number` and
`variant` (cpu/gpu) in the dispatch payload, and adds a cleanup dispatch
on PR close/unlabel.

### Changes

- **`cloud-dispatch-build.yaml`** — Add `pr_number` and `variant` to the
`frontend-asset-build` dispatch payload. Variant is derived from which
preview label triggered the event (`preview-cpu` → cpu, else gpu).

- **`cloud-dispatch-cleanup.yaml`** (new) — Fire-and-forget dispatch of
`frontend-preview-cleanup` to the cloud repo when a frontend PR is
closed or has its preview label removed. Enables synchronized teardown.

### Companion PR

Cloud-side: Comfy-Org/cloud (creates the `deploy-frontend-preview` job,
extends the reconciler)

### How it works

1. Label a frontend PR with `preview`, `preview-cpu`, or `preview-gpu`
2. Assets build and upload to GCS (existing flow)
3. Cloud deploys a full ephemeral env at `fe-pr-{N}.testenvs.comfy.org`
using all `:main` service tags
4. Subsequent pushes update the frontend SHA via AppSet upsert
5. On close/unlabel, cleanup dispatch triggers immediate teardown

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9715-feat-dispatch-frontend-PR-preview-environments-to-cloud-31f6d73d3650819da1b5ca5ce419e06e)
by [Unito](https://www.unito.io)

.github/workflows/cloud-dispatch-cleanup.yaml

@@ -0,0 +1,39 @@
+---
+# Dispatches a frontend-preview-cleanup event to the cloud repo when a
+# frontend PR with a preview label is closed or has its preview label
+# removed. The cloud repo handles the actual environment teardown.
+#
+# This is fire-and-forget — it does NOT wait for the cloud workflow to
+# complete. Status is visible in the cloud repo's Actions tab.
+
+name: Cloud Frontend Preview Cleanup Dispatch
+
+on:
+  pull_request:
+    types: [closed, unlabeled]
+
+permissions: {}
+
+jobs:
+  dispatch:
+    # Only dispatch when:
+    # - PR closed AND had a preview label
+    # - Preview label specifically removed
+    if: >
+      github.repository == 'Comfy-Org/ComfyUI_frontend' &&
+      ((github.event.action == 'closed' &&
+        (contains(github.event.pull_request.labels.*.name, 'preview') ||
+         contains(github.event.pull_request.labels.*.name, 'preview-cpu') ||
+         contains(github.event.pull_request.labels.*.name, 'preview-gpu'))) ||
+       (github.event.action == 'unlabeled' &&
+        contains(fromJSON('["preview","preview-cpu","preview-gpu"]'), github.event.label.name)))
+    runs-on: ubuntu-latest
+    steps:
+      - name: Dispatch to cloud repo
+        uses: peter-evans/repository-dispatch@28959ce8df70de7be546dd1250a005dd32156697 # v4.0.1
+        with:
+          token: ${{ secrets.CLOUD_DISPATCH_TOKEN }}
+          repository: Comfy-Org/cloud
+          event-type: frontend-preview-cleanup
+          client-payload: >-
+            {"pr_number": "${{ github.event.pull_request.number }}"}