ci: make required Frontend check report on every PR

[mergemaven11]

Problem

Test, lint and build frontend is now a globally required branch-protection check, but Frontend CI only triggers when frontend files change. Docs-only or backend-only PRs can therefore remain blocked waiting for a required check that never starts.

Fix

• Trigger Frontend CI on every pull request so the required check always reports.
• Detect whether the PR actually changes frontend/** or the Frontend CI workflow itself.
• Run the full Node/browser/lint/build/bundle suite only for frontend-relevant PRs.
• For non-frontend PRs, complete the same required job successfully after a lightweight change-detection step.
• Keep push to main path-filtered so unnecessary post-merge frontend work is not added.

Why this preserves protection

The globally required job still exists and reports on every PR. Frontend changes still run the full existing validation suite, including the real-Chrome click audit. Non-frontend changes no longer wait indefinitely or spend several minutes running irrelevant browser tests.

Merge gate

Do not merge unless Backend, Frontend, and Security checks are green on this exact head, the branch is current with main, the PR is mergeable, and there are no unresolved blocker comments/reviews.