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_selection_area.dart

@@ -33,11 +33,7 @@ class _ClideSelectionAreaState extends State<ClideSelectionArea> {
   @override
   Widget build(BuildContext context) {
     return DefaultTextEditingShortcuts(
-      child: SelectableRegion(
-        focusNode: _focusNode,
-        selectionControls: emptyTextSelectionControls,
-        child: widget.child,
-      ),
+      child: SelectableRegion(focusNode: _focusNode, selectionControls: emptyTextSelectionControls, child: widget.child),
     );
   }
 }