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/ipc_status/src/extension.dart

@@ -10,11 +10,5 @@ class IpcStatusExtension extends ClideExtension {
   String get version => '0.2.0';
 
   @override
-  List<ContributionPoint> get contributions => [
-        StatusItemContribution(
-          id: 'ipc-status.indicator',
-          priority: 100,
-          build: (_) => const ToolStatusItem(),
-        ),
-      ];
+  List<ContributionPoint> get contributions => [StatusItemContribution(id: 'ipc-status.indicator', priority: 100, build: (_) => const ToolStatusItem())];
 }

lib/builtin/ipc_status/src/status_item.dart

@@ -36,7 +36,11 @@ class ToolStatusItem extends StatelessWidget {
       child: Row(
         mainAxisSize: MainAxisSize.min,
         children: [
-          Container(width: 7, height: 7, decoration: BoxDecoration(color: color, shape: BoxShape.circle)),
+          Container(
+            width: 7,
+            height: 7,
+            decoration: BoxDecoration(color: color, shape: BoxShape.circle),
+          ),
           const SizedBox(width: 6),
           ClideText(label, fontSize: clideFontCaption, color: color),
         ],