fix unhandled exception crash when sharing WAL audio file

[krushnarout]

Problem

Crashlytics reported a fatal crash when sharing WAL audio files:

Fatal Exception: FlutterError - Exception: Unable to create shareable audio file
  AudioPlayerUtils.shareAsAudio (audio_player_utils.dart:200)
  SyncProvider.shareWalAsWav (sync_provider.dart:553)
  _WalItemDetailPageState._handleShare (wal_item_detail_page.dart:604)

Crashlytics: https://console.firebase.google.com/u/0/project/based-hardware/crashlytics/app/ios:com.friend-app-with-wearable.ios12/issues/f0be1b339c31954195a689fc6c4765ff

_handleShare had a try/finally but no catch — the exception thrown by shareAsAudio when the audio file can't be created propagated as an unhandled FlutterError and crashed the app.

Fix

• Add a catch block in _handleShare to show a snackbar error instead of crashing
• Remove dead _isSharing field and its two setState calls — the field was written but never read in the UI

Test plan

• Share a WAL audio file — works normally
• Simulate failure (corrupted/missing file) — shows error snackbar, no crash
• Verify Crashlytics issue stops reproducing after rollout

🤖 Generated with Claude Code

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}

[kodjima33]

mobile bug fix conf 5/5: catch share failure (was uncaught FlutterError crash) + show error snackbar