Fix frontend E2E coverage and gate seeded tests

[romanlutz]

Summary

• stabilize mock, seeded, and live Playwright coverage against the current frontend and backend contracts
• correct Responses API image serialization and cover it with unit and Entra-authenticated integration tests
• add mock and seeded Playwright suites, strict skip/flaky enforcement, and the production frontend build to PR CI

Validation

• Jest: 833 passed
• Playwright mock: 62 passed
• Playwright seeded: 61 passed
• Playwright live: 24 passed
• Responses target unit tests: 68 passed
• lint, type-check, production build, Ruff, ty, and pre-commit passed