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/keybindings_ui/src/extension.dart

@@ -28,24 +28,19 @@ class KeybindingsUiExtension extends ClideExtension {
 
   /// Presets that ship today, each exposed as a `keymap.preset.<name>`
   /// command that activates it.
-  static const _presets = <String, String>{
-    'default': 'Keymap: Default',
-    'vim': 'Keymap: Vim',
-    'vscode': 'Keymap: VS Code',
-    'jetbrains': 'Keymap: JetBrains',
-  };
+  static const _presets = <String, String>{'default': 'Keymap: Default', 'vim': 'Keymap: Vim', 'vscode': 'Keymap: VS Code', 'jetbrains': 'Keymap: JetBrains'};
 
   @override
   List<ContributionPoint> get contributions => [
-        for (final entry in _presets.entries)
-          CommandContribution(
-            id: 'keymap.preset.${entry.key}',
-            command: 'keymap.preset.${entry.key}',
-            title: entry.value,
-            run: (_) async {
-              await _keymap?.setPreset(entry.key);
-              return IpcResponse.ok(id: '', data: {'preset': entry.key});
-            },
-          ),
-      ];
+    for (final entry in _presets.entries)
+      CommandContribution(
+        id: 'keymap.preset.${entry.key}',
+        command: 'keymap.preset.${entry.key}',
+        title: entry.value,
+        run: (_) async {
+          await _keymap?.setPreset(entry.key);
+          return IpcResponse.ok(id: '', data: {'preset': entry.key});
+        },
+      ),
+  ];
 }