[P6.5.1] frontend-tests CI: add npm run build + re-enable frontend lint

[frankbria]

Problem

Two CI safety-net gaps for the actively-developed web UI:

1. CI never runs npm run build. The frontend-tests job runs npm run test:coverage only; npm run build appears only in deploy.yml (deploy time). A TypeScript/build break passes PR CI and only fails at deploy.
2. Frontend lint is disabled in CI with a comment calling web-ui "legacy" — but CLAUDE.md states the web UI is "actively developed — not legacy" and mandates not leaving CI gates disabled in active areas. Lint currently surfaces 5 errors, including a dead import in shipped code: web-ui/src/components/proof/CaptureGlitchModal.tsx:7 ('Input' is defined but never used).

Evidence

• .github/workflows/test.yml:248-289 — frontend-tests runs coverage only, no build
• .github/workflows/test.yml:51-52 — # Note: Frontend linting disabled - web-ui is legacy during v2 CLI-first refactor
• eslint . → 5 errors / 7 warnings; the one shipped-code error is CaptureGlitchModal.tsx:7

Fix

• Add a npm run build step to the frontend-tests job.
• Re-enable frontend lint in code-quality.
• Clear the 5 lint errors (remove dead import in CaptureGlitchModal.tsx; fix unused imports in the named test files).

Acceptance criteria

• frontend-tests runs npm run build and fails the PR on a build break.
• Frontend lint runs in CI and is green.
• frontend-tests remains wired into test-summary.

Source: release-readiness audit 2026-06-13 (frontend + test/CI agents).

[coderabbitai]

🔗 Related PRs

#36 - fix: Add npm install step for E2E test dependencies [merged]
#93 - fix(e2e): Fix dashboard and metrics tests for tab-based UI rendering [merged]
#182 - fix(checkpoints): Unwrap API response to prevent sort error on empty projects [merged]
#189 - ci: Add npm security audit gate to deployment pipelines [merged]
#539 - docs: add CI gate process rule to CLAUDE.md [merged]

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