Add GPL-3.0 license and credits

Required due to bundling imessage-database (GPL-3.0 licensed)

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

Co-Authored-By: Claude <[email protected]>

Author: ndbroadbent

CREDITS.md

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+# Credits
+
+ChatToMap Desktop is built on the shoulders of these open source projects:
+
+## Core Dependencies
+
+### imessage-exporter / imessage-database
+- **Author**: Christopher Sardegna (ReagentX)
+- **Repository**: https://github.com/ReagentX/imessage-exporter
+- **License**: GPL-3.0
+- **Purpose**: Core iMessage database reading and export functionality
+
+### Tauri
+- **Organization**: Tauri Programme within The Commons Conservancy
+- **Repository**: https://github.com/tauri-apps/tauri
+- **License**: Apache-2.0 / MIT
+- **Purpose**: Desktop application framework
+
+## Why GPL-3.0?
+
+This project is licensed under GPL-3.0 because it incorporates `imessage-database`, which is GPL-3.0 licensed. Per the GPL-3.0 license terms, any work that includes GPL code must also be licensed under GPL-3.0.
+
+## Contributing
+
+Contributions are welcome! By contributing to this project, you agree to license your contributions under GPL-3.0.