feat: support multiple PDF uploads per generation

[YizukiAme]

Summary

Adds support for uploading multiple PDFs per generation session. Currently only a single PDF can be uploaded — this PR extends the full pipeline to handle up to 5 PDFs with parallel parsing, intelligent content aggregation, and graceful failure recovery.

Closes #148

Key Changes

Upload Layer

• pdfFile: File | null → pdfFiles: SelectedPdf[]
• Toolbar supports multi-select, drag-and-drop of multiple files
• File deduplication by name + size + lastModified signature
• Validation: single file 50MB, max 5 files, total 150MB

Document Aggregation (lib/pdf/document-aggregator.ts — NEW)

• Text budget allocation: "base guarantee + proportional" split of MAX_PDF_CONTENT_CHARS (50000) across files, after dynamically deducting header/separator framing chars
• Stable image ID rewriting: img_${fileOrder}_${localIndex} intermediate IDs → flattened to sequential global IDs. Pure function, no module-level state, retry-safe
• Vision pool: round-robin selection across files (1 per file first, then fill remaining), sorted by visionPriority

Generation Preview

• Parallel PDF parsing with configurable concurrency (default: 2)
• Per-file status display: pending → parsing → success/failed
• Partial failure recovery: "Retry failed" or "Continue with successful files"
• Backward compatible with legacy single-file sessions

Downstream

• sortPdfImagesForVision() integrated into outline-generator, scene-generator, and outlines-stream route
• No changes to /api/parse-pdf protocol — all multi-file semantics handled client-side

Files Changed (12)

File	Change
lib/pdf/document-aggregator.ts	NEW — core aggregation logic (248 lines)
lib/types/generation.ts	Added SelectedPdf, SessionPdfSource, PdfParseStatus
lib/utils/image-storage.ts	Added storePdfFiles() helper
lib/i18n/common.ts	Added multi-PDF toolbar strings (zh/en)
lib/i18n/generation.ts	Added parsing progress, failure, retry strings
app/page.tsx	Multi-file state management + session construction
components/generation/generation-toolbar.tsx	Multi-select UI, file list, validation
app/generation-preview/page.tsx	Parallel parse + aggregate + failure gate
app/generation-preview/types.ts	Added pdfSources to session state
lib/generation/outline-generator.ts	sortPdfImagesForVision integration
lib/generation/scene-generator.ts	sortPdfImagesForVision integration
app/api/generate/scene-outlines-stream/route.ts	sortPdfImagesForVision integration

Testing

• ✅ prettier — all files pass
• ✅ tsc --noEmit — clean (only pre-existing .next/ validator errors)
• ✅ Manual test: uploaded 4 PDFs (math × 2, physics, chemistry), generation completed successfully with correct per-file content separation in output