Dissolve the model-store normalized cache

Part of #1946 — Redux → React Query migration (Stage 1).

Goal: delete the normalized models cache now that React Query is the cache.

Tasks

• After the feature conversions, audit remaining state.models.* selectors (generic/model-store, src/data/hooks.ts useContextId, …) and move any residual reads to query data.
• Provide a small replacement for useContextId (read the active courseId from route/params or a context) — it currently reads courseware / courseHome Redux state.
• Delete the generic/model-store slice and the models reducer.

Verify: no state.models references remain; full suite green.

Note (size): most useModel readers (204 sites / 82 files today) fall away as courseware converts (#1976 and its siblings, esp. the metadata/sequence layers, keep them alive via the model-store bridge). Reassess this issue's residual scope once the courseware decomposition lands — it may split (e.g. useContextId replacement vs. bridge removal). The useContextId replacement also overlaps with the courseware teardown (#1976); coordinate so it isn't done twice.