test: add cross-route theme contrast release gate

[ShuhaoZhangTony]

Closes #14

What changed

• split cross-route theme roles into explicit surface/text/border tokens for info, success, warning, and error states
• migrated onboarding, Support/references, notification states, settings/account cards, workflow states, and operational list cards away from light-only hardcoded colors
• fixed recommendation/settings-card boundary contrast and added an explicit accessible account-settings name
• added alpha-composited computed-style audits across dark/light × desktop/390x844 for chat, completed Support answer, status, settings, account, and success/warning/error states
• added four reviewed Firefox screenshot baselines and CI-uploaded baseline/diff artifacts
• documented the theme contract and new-component release checklist
• bumped release to v4.6.29 and static assets to app/styles 4221

Verification

• npm run test:browser: 15 passed
• uv run pytest tests/test_frontend_contract.py -q: 17 passed
• full backend CI-equivalent suite: 706 passed, 28 skipped
• uv run ruff check src/ tests/: passed
• desktop and 390x844 dark/light screenshots visually inspected