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/widgets/src/clide_pane.dart

@@ -16,13 +16,7 @@ import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 
 class ClidePane extends StatefulWidget {
-  const ClidePane({
-    super.key,
-    required this.contributionId,
-    required this.child,
-    this.statusWidget,
-    this.active = true,
-  });
+  const ClidePane({super.key, required this.contributionId, required this.child, this.statusWidget, this.active = true});
 
   /// The workspace contribution id this pane belongs to (what
   /// [FocusTracker.activeContributionId] reports when it's focused).