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/markdown/src/extension.dart

@@ -19,14 +19,14 @@ class MarkdownExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        TabContribution(
-          id: 'markdown.viewer',
-          slot: Slots.contextPanel,
-          title: 'Markdown',
-          icon: PhosphorIcons.byName('file-text'),
-          build: (_) => const MarkdownViewer(),
-        ),
-      ];
+    TabContribution(
+      id: 'markdown.viewer',
+      slot: Slots.contextPanel,
+      title: 'Markdown',
+      icon: PhosphorIcons.byName('file-text'),
+      build: (_) => const MarkdownViewer(),
+    ),
+  ];
 
   @override
   Future<void> activate(ClideExtensionContext ctx) async {

lib/builtin/markdown/src/markdown_viewer.dart

@@ -95,18 +95,12 @@ class _MarkdownViewerState extends State<MarkdownViewer> {
       return Padding(padding: const EdgeInsets.all(12), child: ClideText(_error!, muted: true));
     }
     if (_content == null) {
-      return const Padding(
-        padding: EdgeInsets.all(12),
-        child: ClideText('Select a .md file to preview it here.', muted: true),
-      );
+      return const Padding(padding: EdgeInsets.all(12), child: ClideText('Select a .md file to preview it here.', muted: true));
     }
     return ClidePaneChrome(
       title: _path ?? 'viewer',
       subtitle: '${_content!.split('\n').length} lines',
-      leading: ReaderPinButton(
-        pinned: _nav?.hasPinned ?? false,
-        onTap: _path != null ? _onPin : null,
-      ),
+      leading: ReaderPinButton(pinned: _nav?.hasPinned ?? false, onTap: _path != null ? _onPin : null),
       trailing: [
         ReaderActionBar(
           canGoBack: _nav?.canGoBack ?? false,