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/terminal/src/extension.dart

@@ -18,14 +18,14 @@ class TerminalExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        TabContribution(
-          id: 'terminal.pane',
-          slot: Slots.workspace,
-          title: 'Terminal',
-          titleKey: 'tab.title',
-          i18nNamespace: id,
-          priority: 100,
-          build: (_) => const TerminalPane(),
-        ),
-      ];
+    TabContribution(
+      id: 'terminal.pane',
+      slot: Slots.workspace,
+      title: 'Terminal',
+      titleKey: 'tab.title',
+      i18nNamespace: id,
+      priority: 100,
+      build: (_) => const TerminalPane(),
+    ),
+  ];
 }

lib/builtin/terminal/src/terminal_pane.dart

@@ -69,13 +69,16 @@ class _TerminalPaneState extends State<TerminalPane> {
     final shell = Platform.environment['SHELL'] ?? '/bin/bash';
     final cwd = Directory.current.path;
 
-    final response = await ipc.request('pane.spawn', args: {
-      'argv': [shell, '-l'],
-      'kind': PaneKind.terminal.wire,
-      'cwd': cwd,
-      'cols': _terminal.viewWidth,
-      'rows': _terminal.viewHeight,
-    });
+    final response = await ipc.request(
+      'pane.spawn',
+      args: {
+        'argv': [shell, '-l'],
+        'kind': PaneKind.terminal.wire,
+        'cwd': cwd,
+        'cols': _terminal.viewWidth,
+        'rows': _terminal.viewHeight,
+      },
+    );
     if (!mounted) return;
     if (!response.ok) {
       setState(() => _error = response.error?.message ?? 'spawn failed');
@@ -120,11 +123,7 @@ class _TerminalPaneState extends State<TerminalPane> {
   void _onTerminalResize(int cols, int rows, int pixelWidth, int pixelHeight) {
     final id = _paneId;
     if (id == null) return;
-    _kernelIpc()?.request('pane.resize', args: {
-      'id': id,
-      'cols': cols,
-      'rows': rows,
-    });
+    _kernelIpc()?.request('pane.resize', args: {'id': id, 'cols': cols, 'rows': rows});
   }
 
   DaemonClient? _kernelIpc() => _kernel()?.ipc;
@@ -144,12 +143,7 @@ class _TerminalPaneState extends State<TerminalPane> {
     return ClidePaneChrome(
       title: 'terminal',
       subtitle: subtitle,
-      child: _error != null
-          ? _ErrorBody(message: _error!)
-          : ClidePtyView(
-              terminal: _terminal,
-              label: 'terminal — $subtitle',
-            ),
+      child: _error != null ? _ErrorBody(message: _error!) : ClidePtyView(terminal: _terminal, label: 'terminal — $subtitle'),
     );
   }
 }
@@ -165,11 +159,7 @@ class _ErrorBody extends StatelessWidget {
         alignment: Alignment.topLeft,
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.start,
-          children: [
-            const ClideText('Terminal unavailable'),
-            const SizedBox(height: 4),
-            ClideText(message, muted: true),
-          ],
+          children: [const ClideText('Terminal unavailable'), const SizedBox(height: 4), ClideText(message, muted: true)],
         ),
       ),
     );