PDF Notes

[Suggested in the discussion linked in the parent issue]

Being able to take notes in PDF files. This would help a lot when using this app for practice because you could mark difficult passages for later practice or add hints on how to play correctly.

No files or tests are named in the issue. Start by scanning the app’s `lib/` area for the PDF view/import flow and any data model or storage code for notes, then locate the relevant screen/widget and persistence entry point. Run `flutter test` to verify baseline health and launch the app to reproduce current PDF behavior. Done means users can add, view, and persist notes associated with a PDF file in normal usage.

由索引模型根據 Issue 內容生成。