Add document assembly mode, configuration save/load, and unit tests

[bencarver]

Summary

This PR adds three major features to Signature Packet IDE:

1. Document Assembly Mode — Match executed/signed PDFs to blank signature pages and assemble final documents
2. Configuration Save/Load — Save entire sessions (extracted pages, edits, matches) with bundled PDFs for instant restore
3. Unit Tests — 13 Vitest tests covering the matching algorithm

Features

Document Assembly

• Executed Page Matching: Upload signed PDFs; AI identifies which signature pages they correspond to
• Auto-Match: Intelligently matches executed pages to blank pages by document name, party, and signatory (Jaccard similarity-based scoring)
• Assembly Progress Grid: Visual checklist organized by signatory (columns) × document (rows)
• Manual Override: Click any cell to manually assign or reassign pages
• Assemble & Download: Generate final PDFs with blank pages swapped for executed counterparts

Configuration Save/Load

• Save Session: Export entire session (extracted pages, user edits, assembly matches) as .json
• Bundled PDFs: Original PDF files embedded as base64 — no re-upload needed on restore
• Instant Restore: Load a saved config and continue working immediately
• Backward Compatible: Old configs without bundled PDFs still load (with "restored" status, prompting re-upload if needed)
• Privacy-First: All processing done in-browser

Testing

• Vitest Integration: Zero-config test runner compatible with Vite
• 13 Unit Tests: Cover matching algorithm comprehensively:
  • Exact matches, fuzzy matching (case/whitespace), containment matching
  • Threshold filtering, greedy uniqueness (each page matched once)
  • User-confirmed/user-overridden match preservation
  • Empty input handling

Changes

• Added Assembly tab in toolbar for assembly mode
• Added Save Config / Load Config buttons in header
• New services: matchingService.ts (with matchingService.test.ts)
• Updated App.tsx with assembly handlers, config save/load logic
• Updated types.ts with assembly-related types (ExecutedUpload, AssemblyMatch, SavedConfiguration)
• New components: CompletionChecklist.tsx, ExecutedPageCard.tsx, MatchPickerModal.tsx
• Updated README.md with Assembly Mode and Save & Restore documentation
• Added .env to .gitignore to prevent API key leaks
• Fixed environment variable references (API_KEY → GEMINI_API_KEY)
• Fixed npm run command in docs (npm start → npm run dev)

Testing

All 13 unit tests pass:

Tested locally with:

• Multiple agreement PDFs
• Executed/signed PDFs with various signature block formats
• Save/load with bundled PDFs
• Assembly and document generation

Notes for Reviewers

• The matching algorithm uses a weighted score (50% doc name, 35% party, 15% signatory) with a 0.4 threshold
• User-confirmed matches are preserved across re-scans
• The save feature includes full assembly state (matches, executed uploads) for complete session restore