refactor: unify video asset readiness and repair warnings

[atomantic]

Video Gen now describes the model, shared encoder, substitute encoder, and IC-LoRA weights in one asset list. One hook derives readiness and independent integrity dismissals, and one banner renderer routes repairs to the matching download. Generate and Add to queue now name the same first missing asset, including substitute encoders.

The model-specific loading gate, off-machine bypass, repair/download suppression, and changed-damage dismissal keys retain their existing behavior. Missing encoder metadata no longer produces literal undefined in repair messages. Repair banners have accessible group names.

Validation:

• Eight rendered-page characterization cases passed before the refactor; eleven cases now cover all four asset mappings, dismissal, repair, gating, tooltip priority, download suppression, and off-machine behavior.
• Focused regression run: 6 files, 40 tests passed; no pre-existing assertions changed.
• Full client suite: 914 files, 11,238 passed and 6 skipped on rerun. The initial run hit an unrelated Beeper cancellation act-warning flake; that file passed alone and the full rerun passed. The subsequent review fixes also passed the focused run above.
• Production client build and changed-file Biome lint passed.
• Claude reviewed locally with medium effort and tools disabled (one configured review round); its metadata finding and concrete coverage findings were addressed and verified locally.

Complexity measured against this branch's actual base (ac2a79c60): VideoGen CC 303 → 218. The same Babel AST counter was used for both: baseline 1 plus conditionals, logical expressions, loops, catch/default patterns, non-default switch cases, and optional member/call branches, excluding nested functions' own paths. This is a fresh measurement rather than the issue's older 328 baseline. The page has zero cached === false checks and no per-asset integrity dismissal states.

Closes #6681