chore(ci): drop auto screenshot refresh job, switch to manual capture (#884)

The release-only `update-screenshots` job opened a `chore/refresh-screenshots`
PR and immediately tried to squash-merge it. Branch protection (1 review,
6 status checks) rejected the merge on every release, leaving an open PR
behind. Screenshots will instead be refreshed manually after UI changes.

Removed:
- The `update-screenshots` job from ci.yml (~57 lines).
- The `paths-ignore: docs/images/**` push trigger filter; its sole purpose
  was to break the auto-merge re-trigger cascade. Its absence also fixes a
  latent bug where docs-only pushes to main would have skipped sync-docs.
- Four `head_ref != 'chore/refresh-screenshots'` guards in other jobs.
- The "doc screenshots" mention in the skip-bot-PRs comment.

Reworked the screenshot capture spec to be opt-in:
- playwright.config.ts now defines two projects. The default `chromium`
  project ignores screenshots.spec.ts; a separate `screenshots` project
  matches it and is invoked manually.
- The e2e CI job runs `--project=chromium` so the screenshots project
  cannot accidentally run in CI.
- Updated the spec's module comment with the new manual invocation.

Net: 86 lines removed, 27 added.

.github/actions/start-app/action.yml

@@ -19,8 +19,8 @@ inputs:
       When 'true', skip running `npm run build` in backend/ and assume the
       caller has already materialized backend/dist (e.g. via download-artifact
       from an upstream job that already built and verified it). Leave 'false'
-      for standalone invocations like the update-screenshots job that do not
-      run in a job graph with a prior build step.
+      for standalone invocations that do not run in a job graph with a
+      prior build step.
     required: false
     default: 'false'