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_icon.dart

@@ -29,12 +29,7 @@ class EmptyIconPainter extends ClideIconPainter {
 }
 
 class ClideIcon extends StatelessWidget {
-  const ClideIcon(
-    this.painter, {
-    super.key,
-    this.size = 14,
-    this.color,
-  });
+  const ClideIcon(this.painter, {super.key, this.size = 14, this.color});
 
   final ClideIconPainter painter;
   final double size;