Add comprehensive project documentation

[sunapi386]

Summary

• Create docs/ directory with 7 documentation files covering the full project
• architecture.md: System design with ASCII diagrams, data flow, storage abstraction, state management, component hierarchy
• setup.md: Prerequisites, installation, environment variables, storage backend configuration
• api-reference.md: All API routes with request/response schemas and examples for both SQLite and Supabase
• components.md: All 11 React components with props interfaces and key behaviors
• utilities.md: All utility modules with function signatures, class methods, and database schema
• features.md: Walkthroughs for PDF upload, keyword search, area selection, OCR, JSON import/export, sidebar navigation

Test plan

• Verify all markdown files render correctly on GitHub
• Verify internal links between docs resolve properly
• Spot-check file path references and type definitions against source code

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