test(infra): cloud Playwright project with @cloud/@oss tagging (#10546)

## What

Adds a `cloud` Playwright project so E2E tests can run against
`DISTRIBUTION=cloud` builds, with `@cloud` / `@oss` test tagging.

## Why

100+ usages of `isCloud` / `DISTRIBUTION` across 9 categories (API
routing, UI visibility, settings, auth). Zero cloud test infrastructure
existed — cloud-specific UI components (LoginButton, SubscribeButton,
etc.) had no E2E coverage path.

## Investigation: Runtime Toggle

Investigated whether `isCloud` could be made runtime-toggleable in
dev/test mode (via `window.__FORCE_CLOUD__`). **Not feasible** —
`__DISTRIBUTION__` is a Vite `define` compile-time constant used for
dead-code elimination. Runtime override would break tree-shaking in
production.

Full investigation:
`research/architecture/cloud-runtime-toggle-investigation.md`

## What's included

### Playwright Config
- New `cloud` project alongside existing `chromium`
- Cloud project: `grep: /@cloud/` — only runs `@cloud` tagged tests
- Chromium project: `grepInvert: /@cloud/` — excludes cloud tests

### Build Script
- `npm run build:cloud` → `DISTRIBUTION=cloud vite build`

### Test Tagging Convention
```typescript
test('works in both', async () => { ... });
test('subscription button visible @cloud', async () => { ... });
test('install manager prompt @oss', async () => { ... });
```

### Example Tests
- 2 cloud-only tests validating cloud UI visibility

## NOT included (future work)
- CI workflow job for cloud tests (separate PR)
- Cloud project is opt-in — not run by default locally

## Unblocks
- Cloud-specific E2E tests for entire team
- TB-03 LoginButton, TB-04 SubscribeButton (@Kaili Yang)
- DLG-04 SignIn, DLG-06 CancelSubscription

Part of: Test Coverage Q2 Overhaul

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10546-test-infra-cloud-Playwright-project-with-cloud-oss-tagging-32f6d73d3650810ebb59dea8ce4891e9)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

.github/workflows/ci-tests-e2e.yaml

@@ -33,6 +33,20 @@ jobs:
           path: dist/
           retention-days: 1
 
+      # Build cloud distribution for @cloud tagged tests
+      # NX_SKIP_NX_CACHE=true is required because `nx build` was already run
+      # for the OSS distribution above. Without skipping cache, Nx returns
+      # the cached OSS build since env vars aren't part of the cache key.
+      - name: Build cloud frontend
+        run: NX_SKIP_NX_CACHE=true pnpm build:cloud
+
+      - name: Upload cloud frontend
+        uses: actions/upload-artifact@v6
+        with:
+          name: frontend-dist-cloud
+          path: dist/
+          retention-days: 1
+
   # Sharded chromium tests
   playwright-tests-chromium-sharded:
     needs: setup
@@ -97,14 +111,14 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        browser: [chromium-2x, chromium-0.5x, mobile-chrome]
+        browser: [chromium-2x, chromium-0.5x, mobile-chrome, cloud]
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6
       - name: Download built frontend
         uses: actions/download-artifact@v7
         with:
-          name: frontend-dist
+          name: ${{ matrix.browser == 'cloud' && 'frontend-dist-cloud' || 'frontend-dist' }}
           path: dist/
 
       - name: Start ComfyUI server