[saffronjam]

Hosts can export an in-progress game as JSON and load it into a paused resume lobby. Players reclaim their saved seats through an invite, game code, or Browse; the host resumes once everyone is ready.

• Add a versioned save document containing game state, logs, and replay history, with strict validation and runtime restoration. Export is host-only because saves contain hidden cards. Bot credentials are excluded.
• Add file and pasted-JSON loading with size and filename limits, concise errors, responsive transitions, and full-screen mobile forms. Resume lobbies show a static roster, owner, and readiness; competing claims show “Seat already taken.”
• Cover round trips, pending selections, continued gameplay, malformed saves, HTTP/WebSocket contracts, concurrent claims, reconnects, and desktop/mobile flows. This supplies the serialization foundation; database persistence and automatic recovery after server restart remain outside this change.

Validation: just prepare-for-commit, frontend production build, the full backend race suite and production-wiring export/import/resume tests, and save/load plus resume-lobby Playwright suites passed (10 desktop and 20 mobile tests, with layout-specific skips). Desktop and iPhone/Android landscape screenshots were inspected; mobile tests use Chromium device emulation.

Closes #212