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/theme_picker/src/extension.dart

@@ -20,46 +20,25 @@ class ThemePickerExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-        // Opens the settings modal (T-238). Command id kept as `theme.pick`
-        // (the welcome theme-link and other callers reference it); ⌘K opens
-        // Settings, whose only section today is the theme picker.
-        CommandContribution(
-          id: 'theme.pick',
-          command: 'theme.pick',
-          title: 'Settings…',
-          defaultBinding: 'ctrl+k',
-          run: _pick,
-        ),
-        // Always-visible switcher in the far-right status bar (T-234).
-        // priority >= 100 places it in the right group; registered after
-        // ipc-status so it sits to its right.
-        StatusItemContribution(
-          id: 'theme-picker.switcher',
-          priority: 110,
-          build: (_) => const ThemeSwitcherStatusItem(),
-        ),
-      ];
+    // Opens the settings modal (T-238). Command id kept as `theme.pick`
+    // (the welcome theme-link and other callers reference it); ⌘K opens
+    // Settings, whose only section today is the theme picker.
+    CommandContribution(id: 'theme.pick', command: 'theme.pick', title: 'Settings…', defaultBinding: 'ctrl+k', run: _pick),
+    // Always-visible switcher in the far-right status bar (T-234).
+    // priority >= 100 places it in the right group; registered after
+    // ipc-status so it sits to its right.
+    StatusItemContribution(id: 'theme-picker.switcher', priority: 110, build: (_) => const ThemeSwitcherStatusItem()),
+  ];
 
   Future<IpcResponse> _pick(List<String> args) async {
     final ctx = _ctx;
     if (ctx == null) {
       return IpcResponse.err(
         id: '',
-        error: IpcError(
-          code: IpcExitCode.toolError,
-          kind: IpcErrorKind.toolError,
-          message: 'theme-picker not activated',
-        ),
+        error: IpcError(code: IpcExitCode.toolError, kind: IpcErrorKind.toolError, message: 'theme-picker not activated'),
       );
     }
-    final selected = await ctx.dialog.show<String>(
-      (context, dismiss) => SettingsView(
-        controller: ctx.theme,
-        onDismiss: dismiss,
-      ),
-    );
-    return IpcResponse.ok(id: '', data: {
-      'selected': selected ?? ctx.theme.currentName,
-    });
+    final selected = await ctx.dialog.show<String>((context, dismiss) => SettingsView(controller: ctx.theme, onDismiss: dismiss));
+    return IpcResponse.ok(id: '', data: {'selected': selected ?? ctx.theme.currentName});
   }
 }

lib/builtin/theme_picker/src/settings_view.dart

@@ -10,11 +10,7 @@ import 'package:flutter/widgets.dart';
 /// (D-69); the shared `theme_families` helpers keep both surfaces in sync.
 /// Selecting a theme applies it live and dismisses; Cancel just closes.
 class SettingsView extends StatefulWidget {
-  const SettingsView({
-    super.key,
-    required this.controller,
-    required this.onDismiss,
-  });
+  const SettingsView({super.key, required this.controller, required this.onDismiss});
 
   final ThemeController controller;
   final void Function([String? selected]) onDismiss;
@@ -233,9 +229,7 @@ class _ThemeRow extends StatelessWidget {
                   )
                 else
                   const SizedBox(width: 20),
-                Expanded(
-                  child: ClideText(displayName, color: fg),
-                ),
+                Expanded(child: ClideText(displayName, color: fg)),
                 ClideText(name, color: tokens.globalTextMuted, fontSize: clideFontCaption),
               ],
             ),

lib/builtin/theme_picker/src/theme_status_item.dart

@@ -129,19 +129,9 @@ class _ThemePopoverState extends State<_ThemePopover> {
         maxWidth: 280,
         maxHeight: 360,
         entries: [
-          ClideMenuItem(
-            label: 'High contrast',
-            active: _hc,
-            keepOpenOnSelect: true,
-            onSelect: _toggleHc,
-          ),
+          ClideMenuItem(label: 'High contrast', active: _hc, keepOpenOnSelect: true, onSelect: _toggleHc),
           const ClideMenuSeparator(),
-          for (final t in _themes)
-            ClideMenuItem(
-              label: t.displayName,
-              active: t.name == currentBase,
-              onSelect: () => _pick(t),
-            ),
+          for (final t in _themes) ClideMenuItem(label: t.displayName, active: t.name == currentBase, onSelect: () => _pick(t)),
         ],
       ),
     );