Dependencies: Flutter upgrade, file_picker 13, flutter_lints 6

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/pubspec.yaml

@@ -18,7 +18,7 @@ dependencies:
     path: ../../packages/core
   sync_relay_server:
     path: ../../packages/server
-  file_picker: ^12.3.0
+  file_picker: ^13.1.0
   hive_ce: ^2.19.3
   hive_ce_flutter: ^2.3.4
   image: ^4.9.2
@@ -32,7 +32,7 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
-  flutter_lints: ^5.0.0
+  flutter_lints: ^6.0.0
 flutter:
   uses-material-design: true
   assets: