feat: add cloud frontend build dispatch workflow (#9308)

## Summary

Adds `.github/workflows/cloud-dispatch-build.yaml` — fires a
`repository_dispatch` event (`frontend-asset-build`) to
`Comfy-Org/cloud` on push to `cloud/*` branches and `main`.

The cloud repo handles the actual build, GCS upload, and secret
management (Sentry, Algolia, GCS creds). This is fire-and-forget.

## Changes

- New workflow: `cloud-dispatch-build.yaml`
- Trigger: `push` to `cloud/*` and `main` only
- Payload: `ref` (commit SHA) + `branch` (branch name), built with `jq`
to prevent injection
- SHA-pinned `peter-evans/repository-dispatch@v4.0.1`
- Hardened: `permissions: {}`, fork guard (`if: github.repository ==
'Comfy-Org/ComfyUI_frontend'`), concurrency to avoid dispatch storms
- `cloud-deploy-frontend.yaml` left unchanged (still needed during
migration)

## Setup Required

A repository secret `CLOUD_DISPATCH_TOKEN` must be configured — see PR
description comments.

## Part of

Frontend separate deploy prep (Task 1.3)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9308-feat-add-cloud-frontend-build-dispatch-workflow-3156d73d36508164a515eb968f6c5d79)
by [Unito](https://www.unito.io)

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

@@ -0,0 +1,45 @@
+---
+# Dispatches a frontend-asset-build event to the cloud repo on push to
+# cloud/* branches and main. The cloud repo handles the actual build,
+# GCS upload, and secret management (Sentry, Algolia, GCS creds).
+#
+# 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 Build Dispatch
+
+on:
+  push:
+    branches:
+      - 'cloud/*'
+      - 'main'
+  workflow_dispatch:
+
+permissions: {}
+
+concurrency:
+  group: cloud-dispatch-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  dispatch:
+    # Fork guard: prevent forks from dispatching to the cloud repo
+    if: github.repository == 'Comfy-Org/ComfyUI_frontend'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Build client payload
+        id: payload
+        run: |
+          payload="$(jq -nc \
+            --arg ref "${GITHUB_SHA}" \
+            --arg branch "${GITHUB_REF_NAME}" \
+            '{ref: $ref, branch: $branch}')"
+          echo "json=${payload}" >> "${GITHUB_OUTPUT}"
+
+      - 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-asset-build
+          client-payload: ${{ steps.payload.outputs.json }}