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>

test/extension/src/contribution_test.dart

@@ -25,10 +25,7 @@ void main() {
     });
 
     test('StatusItemContribution pins the statusbar slot', () {
-      final s = StatusItemContribution(
-        id: 'ipc-status.indicator',
-        build: (_) => const SizedBox.shrink(),
-      );
+      final s = StatusItemContribution(id: 'ipc-status.indicator', build: (_) => const SizedBox.shrink());
       expect(s.slot, Slots.statusbar);
     });
 
@@ -42,20 +39,12 @@ void main() {
     });
 
     test('TrayItemContribution pins the tray slot', () {
-      final t = TrayItemContribution(
-        id: 't',
-        label: 'Label',
-        onSelected: () {},
-      );
+      final t = TrayItemContribution(id: 't', label: 'Label', onSelected: () {});
       expect(t.slot, Slots.tray);
     });
 
     test('ToolbarButtonContribution pins the toolbar slot', () {
-      final b = ToolbarButtonContribution(
-        id: 'save',
-        label: 'Save',
-        onPressed: () {},
-      );
+      final b = ToolbarButtonContribution(id: 'save', label: 'Save', onPressed: () {});
       expect(b.slot, Slots.toolbar);
     });
   });