feat: add trackcollection import

- [x] only for Maestro ? or maybe some for Minstrel too ?
- [x] add UI
- [ ] add some collections pre tagged

No files, tests, or routes are specified. Start by finding where track/collection import behavior is currently implemented in the UI and where Maestro/Minstrel modes are defined. Trace the data flow for collections (including any seed/default data) to decide how to support pre-tagged collections, then add the UI path and behavior there. Consider it done when the import flow is implemented for the chosen scope and validated by existing tests or a clear manual verification in the app.

索引モデルが issue の本文から書いたものです。