chore: upgrade to riverpod 3.x and freezed 3.x

- Upgrade flutter_riverpod to 3.1.0, riverpod_annotation to 4.0.0
- Upgrade freezed to 3.2.3, freezed_annotation to 3.1.0
- Migrate freezed classes to use sealed keyword (freezed 3.x)
- Update provider naming (*NotifierProvider → *Provider)
- Add legacy.dart import for StateNotifierProvider compatibility
- Fix valueOrNull → value for AsyncValue
- Remove unused imports and fields
- Add sync from Authentik button to users/groups pages
- Suppress invalid_annotation_target warning in analysis_options

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/shared/components/data_grid/data_grid.dart

@@ -1,9 +1,9 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_riverpod/legacy.dart';
 
 import 'data_grid_config.dart';
 import 'data_grid_provider.dart';
-import 'data_grid_source.dart';
 import 'data_grid_state.dart';
 import 'widgets/data_grid_bulk_actions.dart';
 import 'widgets/data_grid_empty_state.dart';

lib/shared/components/data_grid/data_grid_exports.dart

@@ -75,6 +75,9 @@
 /// ```
 library;
 
+export 'package:flutter_riverpod/legacy.dart'
+    show StateNotifierProvider, StateNotifier;
+
 export 'data_grid.dart';
 export 'data_grid_action.dart';
 export 'data_grid_column.dart';

lib/shared/components/data_grid/data_grid_provider.dart

@@ -1,7 +1,7 @@
 import 'dart:async';
 
 import 'package:flutter/foundation.dart';
-import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_riverpod/legacy.dart';
 
 import 'data_grid_config.dart';
 import 'data_grid_source.dart';

lib/shared/components/data_grid/data_grid_state.dart

@@ -4,7 +4,7 @@ part 'data_grid_state.freezed.dart';
 
 /// State for a DataGrid instance.
 @freezed
-class DataGridState<T> with _$DataGridState<T> {
+sealed class DataGridState<T> with _$DataGridState<T> {
   const factory DataGridState({
     /// Current items being displayed.
     @Default([]) List<T> items,