[CI] Frontend tests run without coverage and are never uploaded to Codecov

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

The frontend CI job runs npm test (vitest run with no coverage), and there is no codecov upload step for the frontend (ci.yml uploads only backend and contracts). .github/codecov.yml defines only backend and contracts flags. The frontend has a test:coverage script that is unused in CI, so frontend coverage is invisible despite Codecov being configured.

Acceptance criteria

• Run frontend tests with vitest run --coverage in CI
• Upload frontend/coverage/lcov.info to Codecov with a frontend flag
• Add a frontend flag/target to .github/codecov.yml

Files to touch

• .github/workflows/ci.yml
• .github/codecov.yml
• frontend/package.json

Out of scope

• Raising backend coverage thresholds
• Adding Playwright E2E