chore: adopt Dart 3.9 toolchain — honest floor + tall-style reformat (T-353)

Raise the declared minimums in pubspec.yaml to what our deps already
require: Flutter >=3.35.0 / Dart >=3.9.0 (was 3.19.0 / 3.5.0). alchemist
0.12 needs Flutter 3.32; Dart 3.9 first ships in Flutter 3.35, so 3.35 is
the binding floor. Pin the exact build toolchain in .fvmrc (Flutter
3.44.1).

Moving to the Dart 3.9 language level switches `dart format` to the new
"tall" style and enables two new lints. This commit is the resulting
mechanical churn, isolated from any behaviour change:
  - whole-tree `dart format` reformat (tall style)
  - `dart fix` for unnecessary_underscores + use_null_aware_elements

No runtime behaviour change; `make test` green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/kernel/src/notify.dart

@@ -6,13 +6,8 @@ enum NotificationLevel { info, warning, error, success }
 
 @immutable
 class ClideNotification {
-  ClideNotification({
-    required this.id,
-    required this.level,
-    required this.message,
-    this.title,
-    this.duration = const Duration(seconds: 4),
-  }) : createdAt = DateTime.now().toUtc();
+  ClideNotification({required this.id, required this.level, required this.message, this.title, this.duration = const Duration(seconds: 4)})
+    : createdAt = DateTime.now().toUtc();
 
   final String id;
   final NotificationLevel level;
@@ -41,20 +36,9 @@ class Notifications extends ChangeNotifier {
     if (_active.length != before) notifyListeners();
   }
 
-  void _push(
-    NotificationLevel level,
-    String message, {
-    String? title,
-    Duration? duration,
-  }) {
+  void _push(NotificationLevel level, String message, {String? title, Duration? duration}) {
     final id = 'n${_seq++}';
-    final n = ClideNotification(
-      id: id,
-      level: level,
-      message: message,
-      title: title,
-      duration: duration ?? const Duration(seconds: 4),
-    );
+    final n = ClideNotification(id: id, level: level, message: message, title: title, duration: duration ?? const Duration(seconds: 4));
     _active.add(n);
     _timers[id] = Timer(n.duration, () => dismiss(id));
     notifyListeners();