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/kernel/src/dialog.dart

@@ -2,10 +2,7 @@ import 'dart:async';
 
 import 'package:flutter/widgets.dart';
 
-typedef DialogBuilder<T> = Widget Function(
-  BuildContext context,
-  void Function([T? result]) dismiss,
-);
+typedef DialogBuilder<T> = Widget Function(BuildContext context, void Function([T? result]) dismiss);
 
 /// Single-at-a-time modal router.
 ///
@@ -70,12 +67,7 @@ class _Queued {
 /// Hosts the current dialog from [DialogRouter]. Place high in the tree
 /// (inside the WidgetsApp) so dialogs overlay every other surface.
 class DialogHost extends StatelessWidget {
-  const DialogHost({
-    super.key,
-    required this.router,
-    required this.child,
-    this.backdropColor = const Color(0xC0000000),
-  });
+  const DialogHost({super.key, required this.router, required this.child, this.backdropColor = const Color(0xC0000000)});
 
   final DialogRouter router;
   final Widget child;
@@ -98,10 +90,7 @@ class DialogHost extends StatelessWidget {
                 child: ColoredBox(
                   color: backdropColor,
                   child: Center(
-                    child: GestureDetector(
-                      onTap: () {},
-                      child: b(ctx, router.dismiss),
-                    ),
+                    child: GestureDetector(onTap: () {}, child: b(ctx, router.dismiss)),
                   ),
                 ),
               ),