chore(flutter): bump version to 2026.5.21+14 for v2026.05.21.0

Last meaningful-feature release on Flutter. Ships:
  - #472: notification + Wear OS controls stay live across idle
    teardown (audio_handler softTeardown split)
  - #479: system-playlist tap surfaces empty / slow / failed
    states with SnackBar feedback
  - #399: drift test cohort re-enabled on ci-flutter:1.26
  - Tier-A deps sweep (audio_session 0.2, flutter_lucide,
    permission_handler), Go server bumps, golangci v2 schema,
    Flutter 3.44 ListTile strictness fixes

Future Flutter releases on this codebase are bugfix-only; the
v1 Android native rewrite has been planned.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/pubspec.yaml

@@ -1,7 +1,7 @@
 name: minstrel
 description: Minstrel mobile client
 publish_to: 'none'
-version: 2026.5.19+13
+version: 2026.5.21+14
 
 environment:
   sdk: '>=3.5.0 <4.0.0'