Add Playwright Storybook visual gate for post-kit-editor

[patoperpetua]

Parent: #138

Goal

Add an InkAds-marketing-style Playwright visual capture + review gate for @singleton-sd/post-kit-editor Storybook stories so PRs that change the admin/canvas UI fail a dedicated visual-review check until a human accepts diffs.

Pattern (from poc-inkads-marketing)

InkAds uses:

• pnpm test:visual — Playwright screenshots + pixelmatch vs baseline → test-results/visual/ (+ manifest.json)
• pnpm test:visual:gate — fails when manifest has changed/new unless VISUAL_ACCEPTED=1 / label visual-accepted
• CI job named visual-review (human gate, not infra failure)

Adapt for PostKit editor:

• Capture Storybook static stories (not a marketing site)
• Baseline: committed baselines under the package or compare PR Storybook build vs base SHA Storybook build (InkAds preferred local VISUAL_BASE_DIR)
• Keep scope small: key stories only (Admin/EmailTemplateAdmin/FullAdmin, Editor/EmailBuilderCanvas/Editable, maybe Editor/EmailTemplateEditor/FullEditor)

Scope

• Scripts on @singleton-sd/post-kit-editor: build-storybook, test:visual, test:visual:gate, playwright:install
• packages/post-kit-editor/tests/visual.mjs + visual-gate.mjs (+ thin report HTML if practical)
• GitHub Actions job visual-review (can be in ci.yml or a dedicated workflow) that:
  1. builds Storybook
  2. runs test:visual
  3. runs test:visual:gate
  4. uploads test-results/visual artifact
  5. honors visual-accepted label / VISUAL_ACCEPTED like InkAds
• Document how to update baselines and accept intentional diffs in package README
• Seed initial baselines from a green local capture

Constraints

• Do not require Azure Static Web Apps hosting for v1 (artifact + gate is enough; hosted report can be follow-up)
• Synthetic fixtures only — no secrets
• Do not block the existing Lint / test / build job on visual flakiness beyond the dedicated visual-review job
• Work in the existing #139 worktree/PR branch if open, or a new worktree from origin/main after merge

Acceptance criteria

• pnpm --filter @singleton-sd/post-kit-editor build-storybook works
• pnpm --filter @singleton-sd/post-kit-editor test:visual writes screenshots + manifest.json
• test:visual:gate fails on changed/new vs baseline when not accepted
• CI exposes a visual-review check
• README documents local + CI acceptance flow

Agent implementation notes

Read ~/src/inkads/marketing/poc-inkads-marketing/main/tests/visual.mjs and visual-gate.mjs plus .github/workflows/preview.yml visual-review job. Adapt Storybook iframe/story URLs (iframe.html?id=...). Branch: feat/<N>-editor-visual-gate or continue on feat/139-email-builder-mui-canvas if this is stacked on #142.