Add Tauri command for AI chat via reqwest:
- ai_chat module with send_chat() using Anthropic Messages API
- ANTHROPIC_API_KEY from environment variable
- Proper error handling for missing key, API errors, parse failures
- Unit tests for request building and response extraction
- Mock handler added to mock-tauri.ts for browser testing
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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