docs: add comprehensive project documentation

- ARCHITECTURE.md: Complete system architecture documentation covering all 10 subsystems, design patterns, data flows, and security architecture
- API.md: Full API reference for all 92 Tauri commands with TypeScript signatures, examples, and usage patterns
- COMPONENTS.md: Comprehensive React component documentation with hierarchy, props, state management, and patterns
- IMPROVEMENTS.md: Prioritized improvement roadmap with code examples, sprint planning, and success metrics

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Sallvainian