refactor(generate): remove frontend video preview & AI title generation, adjust step3 layout
Task 1 - Delete all frontend video preview code:
- Remove FrontendPreviewPlayer, CanvasPreviewGrid, PreviewControls, PreviewProgressBar
- Remove usePreviewAssets, usePreviewAudio, useVariantVoicePreview, useSegmentScheduler hooks
- Clean up GeneratePage: remove preview state/effects/callbacks/render block
- Remove onServerClipsChange prop chain (GenerateStepContent → Step2 still optional)
- Layout: currentStep===3 now uses full-width (no more step4-layout dual column)
Task 2 - Delete AI title generation:
- Remove AiTitleGenerator, AiTitleCard components
- Remove useAiTitleGenerator hook
- Remove AI_TITLE_TEMPLATES from constants.ts
- Remove AiTitleResult type from types.ts
- Clean up Step4TitleSettings: remove AI toggle, AI generate buttons, batch AI row
- Clean up useStep4Title: remove all aiTitle* fields and auto-generate effect
Task 3 - Step3 title page layout adjustment:
- generate.css: remove step4-layout grid, preview-col rules, title-sidebar max-height
- Step4TitleSettings: title inputs max-width 640px, template grid uses full width
- TitleStylePanel with enableTemplates gets 4-5 columns via auto-fill responsive grid
Preserved: useBatchVariantPlans (used by useGenerateVideo), PreviewCountModal,
aiAutoSelect field in TitleSettings (used by buildPayload/ai-avatar), TitleMiniPreview
(shared with TitleStylePanel for AI数字人)
xiaoxia
eeebdbfdc9