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/widgets/src/clide_divider.dart

@@ -2,11 +2,7 @@ import 'package:clide/kernel/src/theme/controller.dart';
 import 'package:flutter/widgets.dart';
 
 class ClideDivider extends StatelessWidget {
-  const ClideDivider({
-    super.key,
-    this.axis = Axis.horizontal,
-    this.thickness = 1,
-  });
+  const ClideDivider({super.key, this.axis = Axis.horizontal, this.thickness = 1});
 
   final Axis axis;
   final double thickness;
@@ -14,10 +10,6 @@ class ClideDivider extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     final color = ClideTheme.of(context).surface.dividerColor;
-    return Container(
-      width: axis == Axis.vertical ? thickness : null,
-      height: axis == Axis.horizontal ? thickness : null,
-      color: color,
-    );
+    return Container(width: axis == Axis.vertical ? thickness : null, height: axis == Axis.horizontal ? thickness : null, color: color);
   }
 }