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_scrollbar.dart

@@ -4,12 +4,7 @@ import 'package:flutter/widgets.dart';
 /// Thin themed scrollbar. Tier-0 shell; more refined scrolling (velocity
 /// multiplier, keyboard nav) lands with the editor.
 class ClideScrollbar extends StatelessWidget {
-  const ClideScrollbar({
-    super.key,
-    required this.controller,
-    required this.child,
-    this.axis = Axis.vertical,
-  });
+  const ClideScrollbar({super.key, required this.controller, required this.child, this.axis = Axis.vertical});
 
   final ScrollController controller;
   final Widget child;
@@ -22,25 +17,13 @@ class ClideScrollbar extends StatelessWidget {
       slider: tokens.scrollbarSlider,
       sliderHover: tokens.scrollbarSliderHover,
       track: tokens.scrollbarTrack,
-      child: RawScrollbar(
-        controller: controller,
-        thumbColor: tokens.scrollbarSlider,
-        thickness: 8,
-        radius: const Radius.circular(4),
-        child: child,
-      ),
+      child: RawScrollbar(controller: controller, thumbColor: tokens.scrollbarSlider, thickness: 8, radius: const Radius.circular(4), child: child),
     );
   }
 }
 
 class ScrollbarTheme extends InheritedWidget {
-  const ScrollbarTheme({
-    super.key,
-    required this.slider,
-    required this.sliderHover,
-    required this.track,
-    required super.child,
-  });
+  const ScrollbarTheme({super.key, required this.slider, required this.sliderHover, required this.track, required super.child});
 
   final Color slider;
   final Color sliderHover;