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/multitab_controller.dart

@@ -5,13 +5,7 @@ import 'package:flutter/foundation.dart';
 /// an editor buffer ref).
 @immutable
 class MultitabEntry<T> {
-  const MultitabEntry({
-    required this.id,
-    required this.title,
-    required this.payload,
-    this.closeable = true,
-    this.reorderable = true,
-  });
+  const MultitabEntry({required this.id, required this.title, required this.payload, this.closeable = true, this.reorderable = true});
 
   /// Stable id for this tab. Must be unique within the controller.
   final String id;
@@ -31,12 +25,7 @@ class MultitabEntry<T> {
   /// reorderable tabs cannot move past a pinned tab on either side.
   final bool reorderable;
 
-  MultitabEntry<T> copyWith({
-    String? title,
-    T? payload,
-    bool? closeable,
-    bool? reorderable,
-  }) {
+  MultitabEntry<T> copyWith({String? title, T? payload, bool? closeable, bool? reorderable}) {
     return MultitabEntry<T>(
       id: id,
       title: title ?? this.title,