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_tappable.dart

@@ -95,10 +95,7 @@ class _ClideTappableState extends State<ClideTappable> {
     child = DecoratedBox(
       position: DecorationPosition.foreground,
       decoration: BoxDecoration(
-        border: Border.all(
-          color: _focused ? tokens.globalFocus : const Color(0x00000000),
-          width: 2,
-        ),
+        border: Border.all(color: _focused ? tokens.globalFocus : const Color(0x00000000), width: 2),
         borderRadius: BorderRadius.circular(3),
       ),
       child: child,
@@ -109,16 +106,8 @@ class _ClideTappableState extends State<ClideTappable> {
     // provider. The reverse nesting would leave Actions as a descendant
     // of the focused context — unreachable.
     child = Actions(
-      actions: <Type, Action<Intent>>{
-        ActivateIntent: CallbackAction<ActivateIntent>(onInvoke: _activate),
-      },
-      child: Focus(
-        focusNode: _effectiveFocus,
-        canRequestFocus: enabled,
-        autofocus: widget.autofocus,
-        onFocusChange: _setFocused,
-        child: child,
-      ),
+      actions: <Type, Action<Intent>>{ActivateIntent: CallbackAction<ActivateIntent>(onInvoke: _activate)},
+      child: Focus(focusNode: _effectiveFocus, canRequestFocus: enabled, autofocus: widget.autofocus, onFocusChange: _setFocused, child: child),
     );
 
     if (widget.tooltip != null) {