Pdf text highlight (#49)

* feat(pdf): implement native text highlighting and interactive selection menu

- Add `PdfUserHighlight` data model to track page index, character ranges, and normalized coordinates.
- Redesign `PdfSelectionMenuPopup` to match the EPUB reader style, featuring a color palette row and a deletion option.
- Integrate highlight rendering into the `PdfPageSelectionsLayer` using Canvas drawing.
- Add hit-testing to `detectTapGestures` to allow users to tap existing highlights to change colors or remove them.

* Implemented PDF highlight persistence, sync, and exporting.

* Improved PDF highlight exporting and visibility logic

- Fixed PDF highlight rendering by correctly calculating coordinates and merging adjacent rectangles into lines.
- Updated `PdfExporter` to include highlights in the exported document and added debug logging.
- Corrected visibility check in `PdfPageComposable` to account for vertical offsets.
- Enhanced `PdfViewerScreen` highlight creation to use merged line rectangles

* Refactored annotation synchronization logic and improved background sync reliability.

- Refactored JSON bundle creation and extraction into reusable helper functions.
- Improved stale file detection by checking all annotation types (ink, text, layout, highlights, text boxes).
- Added background annotation downloading to `SyncWorker`
- Implemented duplicate file cleanup in `GoogleDriveRepository` during uploads.
- Added a `showFeedback` flag to `syncFolderMetadata` to control UI visibility during background operations.
- Fixed a potential data loss edge case by aborting Firestore sync if the bundle upload fails.

* Improved highlight interaction and management in PDF viewer

- Implemented hit tolerance for highlight selection to improve touch detection accuracy.
- Added logic to shift highlight page indices when adding or deleting pages.
- Included highlights in the auto-pruning check for empty pages.
- Added a "Highlights" tab to the navigation drawer to list, navigate to, and delete user highlights.

LICENSE

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