[talker_flutter] remove share_plus dependency to eliminate Flutter KGP warning

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talker_flutter currently pulls share_plus, whose Android plugin applies kotlin-android directly and triggers Flutter’s KGP warning. This change removes that dependency path and keeps log export functionality by saving logs locally to temporary storage instead of sharing via share_plus.

• Dependency graph

  • Removed share_plus and path_provider from packages/talker_flutter/pubspec.yaml to stop pulling the offending Android plugin chain.

• Log export flow

  • Native export now writes logs to system temp and returns saved file path.
  • Web export now writes a timestamped file name and returns that saved location identifier.

• Controller + UI alignment

  • TalkerViewController now stores the last saved logs file location.
  • Action label updated from sharing semantics to local-save semantics.
  • Snackbar now reports saved location (with fallback message).
  • Save failures are surfaced via widget.talker.handle(error, stackTrace) and user-facing failure toast.

• Package docs

  • Added a CHANGELOG.md entry describing removal of share_plus to avoid Flutter KGP warning.

Future<String> downloadFile(String logs, DateTime timestamp) async { ... }

await _controller.downloadLogsFile(logs);
final fileLocation = _controller.lastSavedLogsFileLocation;
_showSnackBar(context, 'Logs file saved: $fileLocation');