Split VideoGen.jsx into hooks + subcomponents

[atomantic]

Summary

Decomposes the ~1990-line client/src/pages/VideoGen.jsx god-component (issue #2834) into focused modules while preserving behavior and the URL/deep-link contract exactly.

• Domain logic → hook. The client-side serial batch-queue orchestration (queue state, running marker, worker effect, BUSY-backoff retry, unmount/staleness guards) moves into client/src/hooks/useVideoGenQueue.js. The page passes { generating, runGeneration } and gets back { queue, enqueue, removeFromQueue, clearFinishedQueue, cancelRunning }. runGeneration is held on a ref inside the hook so the worker effect keeps its original [queue, generating, runningQueueId] deps and never re-dispatches on the page's per-render closure churn.
• Pure helpers → lib. videoModelMemoryGb, computeFflfSafeFrames (FFLF/ltx2 pixel-budget back-solve, mirrors server/services/videoGen/local.js), isModelAllowedForMode, and the FRAME_OPTIONS/FPS_OPTIONS/VIDEO_EDGE_BOUNDS constants move to client/src/lib/videoGenParams.js (per CLAUDE.md: side-effect-free helpers live in lib/, not hooks/). Added with unit tests.
• UI → subcomponents under client/src/components/videoGen/: RuntimeFingerprint, ModelRepairBanner (deduped — one component now serves both the model and text-encoder integrity banners), VideoPreviewPanel, and VideoGenGallery (recent + hidden sections).
• fetch('/api/...') boundary leak at the old VideoGen.jsx:876 runtime-status probe now routes through a new getVideoGenRuntimeStatus(runtime, { signal }) wrapper in client/src/services/apiImageVideo.js (silent, honors the AbortController signal — same swallow-on-failure behavior).
• Accessibility: the Seed <input> gains a proper <label htmlFor="video-seed">/id pairing.

New public modules are registered in the client/src/lib and client/src/hooks barrels + README catalogs (enforced by index.test.js). Net −396 lines from the page.

Test plan

• client ESLint clean on all touched files.
• New client/src/lib/videoGenParams.test.js (13 cases) passes.
• client/src/lib/index.test.js + client/src/hooks/index.test.js (barrel/README enforcement) pass.
• client/src/components/videoGen/ existing component tests pass.
• Full cd client && npm test suite: 4554 passing; the single failure (StoryBuilder.test.jsx analyzeImport providerOverride) is a pre-existing order-dependent flake — passes in isolation and is unrelated to this change.
• vite build succeeds.

Closes #2834