Pack library UI: owned premium DLC playable, unowned available-to-buy

[charneykaye]

Pack library UI: surface owned premium DLC as playable and unowned as available-to-buy.

The ownership primitive (useSteamDlc / useSteamDlcOwned) is in place, but the Scenario Library / pack browser does not yet consume it. Per docs/DLC_MODEL.md, the app should:

• Discover installed premium packs at the DLC install path (e.g. dlc/<pack_id>/) and show owned ones as playable.
• Show unowned premium packs as available to buy, with a link to that DLC's Steam store page (open via the Steam overlay when running under Steam).
• Never hide premium packs or hard-error when a DLC is absent — an "available to buy" affordance is the only gating.
• Keep the four official packs unconditionally free and playable.

References

• docs/DLC_MODEL.md — "How the app unlocks owned DLC"
• apps/web/src/hooks/useSteamDlc.ts
• apps/web/src/screens/ScenarioLibrary.tsx

Depends on the Steam registration + CI wiring issues.