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/builtin/git/src/extension.dart

@@ -16,20 +16,16 @@ class GitExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        TabContribution(
-          id: 'git.panel',
-          slot: Slots.sidebar,
-          title: 'Git',
-          icon: PhosphorIcons.byName('git-branch'),
-          titleKey: 'tab.title',
-          i18nNamespace: id,
-          priority: -80,
-          build: (_) => const GitPanelView(),
-        ),
-        StatusItemContribution(
-          id: 'git.branch',
-          priority: 10,
-          build: (_) => const GitStatusItem(),
-        ),
-      ];
+    TabContribution(
+      id: 'git.panel',
+      slot: Slots.sidebar,
+      title: 'Git',
+      icon: PhosphorIcons.byName('git-branch'),
+      titleKey: 'tab.title',
+      i18nNamespace: id,
+      priority: -80,
+      build: (_) => const GitPanelView(),
+    ),
+    StatusItemContribution(id: 'git.branch', priority: 10, build: (_) => const GitStatusItem()),
+  ];
 }