feat(ui): code coverage reporting for ui

[siddhant-galileo]

Summary

• What changed and why: Added Playwright-based UI coverage generation (HTML + LCOV) and wired it into CI/Codecov to track frontend test coverage, filtered to ui/src/**.

Scope

• User-facing/API changes:
  • None (testing/coverage only).
• Internal changes:
  • Playwright fixture now records V8 coverage for Chromium runs.
  • New scripts/coverage-playwright.cjs converts raw coverage to Istanbul HTML + coverage-playwright/lcov.info (only ui/src/**).
  • New test:integration:coverage script and CI step upload UI LCOV to Codecov.
• Out of scope:
  • No changes to backend coverage setup or non-UI tests.

Risk and Rollout

• Risk level: low
• Rollback plan: Revert Playwright fixture coverage hooks, remove coverage-playwright script and test:integration:coverage usage in CI.

Testing

• Added or updated automated tests (kept existing Playwright suite green after fixture changes)
• Ran make check (only UI/Playwright and coverage flow exercised locally)
• Manually verified coverage artifacts (coverage-playwright/index.html and lcov.info generated correctly)

Checklist

• Linked issue/spec (if applicable)
• Updated docs/examples for coverage usage (optional follow-up)
• Included any required follow-up tasks

[codecov]

The author of this PR, siddhant-galileo, is not an activated member of this organization on Codecov.
Please activate this user on Codecov to display this PR comment.
Coverage data is still being uploaded to Codecov.io for purposes of overall coverage calculations.
Please don't hesitate to email us at support@codecov.io with any questions.