fix(version-picker): disable versions missing from RomM

[danielcopper]

What this is

PR 3 of #1570. Retained local ROM rows no longer masquerade as live, selectable versions after RomM drops their ids. Availability is recomputed whenever the Game Page lazily loads the version list; it is never persisted.

What changed

• The bound ROM detail request establishes whether the active id is still available and supplies RomM's direct sibling view.
• Local members absent from that direct view are confirmed concurrently through exact-id, single-attempt get_rom probes. If the bound id itself returns 404, every other local member is probed individually.
• Only RommNotFoundError marks a version vanished; every non-404 failure fails open.
• Vanished rows remain visible with their local state, but are dimmed, labelled No longer available on RomM, disabled, and excluded from Default ranking.
• vanished remains separate from switchable, preserving sibling-group membership semantics.
• A bound 404 returns bound_vanished: true without reporting RomM offline or updating the global connection state.
• SavesTab excludes vanished versions from its stranded-save advice while continuing to inspect live inactive installs.
• The deferred artwork cleanup now reports a missing/falsy ROM as not_found instead of server_unreachable; artwork remains data-only and is not used as liveness evidence.

The version-switch commit guard and deletion/purge behavior remain in PRs 4 and 5.

Safety

Liveness probes use the existing three-second request_once path on the executor fan-out, so they neither retry nor block the event loop. Direct sibling presence is a positive server statement; absence alone never proves disappearance because sibling groups can be transitive.

A restored database or visibility change can make the same id available again. The next list load recomputes the result and removes vanished automatically.

Docs

Updated the backend and Steam-shortcut architecture pages plus the managing-games and save-sync user guides. Frozen ADR history remains unchanged.

Verification

mise run gate passed on the final code state:

• 6334 backend tests
• 2244 frontend tests
• Ruff and basedpyright clean
• TypeScript, ESLint, Prettier, Deno, build, size limit, import contracts, and repository invariant checks passed

The subsequent documentation wording correction passed focused Markdown formatting, link, whitespace, backend, frontend, and 404-classification checks. Final review found no remaining issues.

On-device verification

Open a Game Page whose shortcut is bound to a RomM id that was removed while a replacement version remains in the local sibling group. The page should render without waiting for liveness probes; the removed version should remain visible but disabled, the live replacement should remain selectable, and RomM must not be reported offline.

Closes part of #1570.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.5% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud