chore(flutter): promote path_provider to direct dep

Already a transitive dep via flutter_secure_storage and flutter_cache_manager.
Promoting to direct so we can import it from album_cover_cache (next
commit) for the lock-screen artwork cache directory.

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

flutter_client/pubspec.yaml

@@ -23,6 +23,7 @@ dependencies:
   package_info_plus: ^8.3.1
   pub_semver: ^2.1.4
   cupertino_icons: ^1.0.8
+  path_provider: ^2.1.5
 
 dev_dependencies:
   flutter_test: