test/add-tests

[Ralex91]

Adds unit, integration and e2e tests, and fixes the bugs they found.

• Vitest for common, socket and web: scoring, game flow, REST API, sockets with a real server, translations (pnpm test)
• Playwright e2e in packages/e2e: games, quiz types, settings, editor, media library, results, branding (pnpm test:e2e)
• Tests and E2E Tests CI jobs with a build check, Playwright browser cache and traces on failure, tests kept out of the Docker build context
• GAME_SPEED env var to speed up game delays, only used by the e2e server
• Testing guide in docs/testing.md: commands, test helpers, how to write an e2e test and the CI jobs

Fixes:

• Answers sent outside of the answer phase are ignored
• A player or manager reconnecting from a new connection takes over the old one
• The waiting room survives a short manager disconnect (30s grace period)
• Answers are revealed without waiting for players who left
• Results show the real correctness and points of each question
• Deleting the open question no longer blocks saving the quizz
• Number settings respect their minimum
• Wrong manager password returns 403 with the right message
• Accessible names on icon-only controls, missing Vietnamese first place rank, clearer media delete message