Fix GPX file opening, share sheet visibility, and add SAF track storage

Intent filter fixes:
- Remove pathSuffix=".gpx" + scheme restriction from the application/octet-stream
  ACTION_VIEW filter; pathSuffix matches the URI path, not the filename, so it
  never fired against file manager content URIs (opaque paths like /provider/uuid).
  The existing display-name check in handleIncomingGpx() is the correct gate.
- Add ACTION_SEND filter for application/octet-stream; Nav was invisible in share
  sheets because most file managers send this type for .gpx files.

SAF-based track storage (API 29+):
- Replace fragile SharedPrefs URI cache + OWNER_PACKAGE_NAME MediaStore query with
  Storage Access Framework as the primary write/read path. The user grants access to
  their tracks folder once via ACTION_OPEN_DOCUMENT_TREE; DocumentFile.listFiles()
  finds all GPX files without relying on MediaStore ownership (which Android clears
  on uninstall). After reinstall, a one-tap "Restore track access" button in Saved
  Tracks re-authorizes the same folder and all tracks immediately reappear.
- MediaStore path (existing) is kept as fallback for tracks saved before this change
- Add SafState enum (CONFIGURED / PERMISSION_LOST / NOT_CONFIGURED) in TrackStorage
- TrackRepository exposes safState() and initSafDirectory() thin wrappers
- MainViewModel exposes safState StateFlow and onSafDirectorySelected()
- SavedTracksFragment observes safState and shows contextual setup / re-auth button
- MainActivity registers safPickerLauncher and exposes launchSafPicker()
- Add androidx.documentfile:documentfile:1.0.1 dependency

https://claude.ai/code/session_01YUbuZNDAoLea4cf9UGQ9qn

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