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_svg_view.dart

@@ -15,9 +15,7 @@ class ClideSvgView extends StatelessWidget {
   Widget build(BuildContext context) {
     Widget child;
     if (assetPath != null) {
-      child = ScalableImageWidget.fromSISource(
-        si: ScalableImageSource.fromSvg(rootBundle, assetPath!),
-      );
+      child = ScalableImageWidget.fromSISource(si: ScalableImageSource.fromSvg(rootBundle, assetPath!));
     } else if (svgString != null) {
       final si = ScalableImage.fromSvgString(svgString!);
       child = ScalableImageWidget(si: si);