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/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),
               ],
             ),