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/vim/src/extension.dart

@@ -61,19 +61,19 @@ class VimExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        _modeCommand('vim.mode.normal', 'Vim: Normal mode', () => _mode?.enterNormal()),
-        _modeCommand('vim.mode.insert', 'Vim: Insert mode', () => _mode?.enterInsert()),
-        _modeCommand('vim.mode.visual', 'Vim: Visual mode', () => _mode?.enterVisual()),
-        StatusItemContribution(
-          id: 'vim.mode',
-          priority: -50, // left group, near the other editor status items
-          listenable: _mode,
-          build: (_) {
-            final m = _mode;
-            return m == null ? const SizedBox.shrink() : VimModeIndicator(service: m);
-          },
-        ),
-      ];
+    _modeCommand('vim.mode.normal', 'Vim: Normal mode', () => _mode?.enterNormal()),
+    _modeCommand('vim.mode.insert', 'Vim: Insert mode', () => _mode?.enterInsert()),
+    _modeCommand('vim.mode.visual', 'Vim: Visual mode', () => _mode?.enterVisual()),
+    StatusItemContribution(
+      id: 'vim.mode',
+      priority: -50, // left group, near the other editor status items
+      listenable: _mode,
+      build: (_) {
+        final m = _mode;
+        return m == null ? const SizedBox.shrink() : VimModeIndicator(service: m);
+      },
+    ),
+  ];
 
   CommandContribution _modeCommand(String id, String title, void Function() apply) {
     return CommandContribution(

lib/builtin/vim/src/vim_mode_indicator.dart

@@ -20,12 +20,7 @@ class VimModeIndicator extends StatelessWidget {
         final tokens = ClideTheme.of(context).surface;
         return Padding(
           padding: const EdgeInsets.symmetric(horizontal: 8),
-          child: ClideText(
-            '-- ${service.mode.label} --',
-            fontFamily: clideMonoFamily,
-            fontSize: clideFontCaption,
-            color: tokens.statusBarForeground,
-          ),
+          child: ClideText('-- ${service.mode.label} --', fontFamily: clideMonoFamily, fontSize: clideFontCaption, color: tokens.statusBarForeground),
         );
       },
     );