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_toast.dart

@@ -79,18 +79,17 @@ class _ClideToastState extends State<ClideToast> {
                 children: [
                   ClideIcon(style.icon, size: clideIconStandard, color: style.color),
                   const SizedBox(width: clideGapStandard),
-                  Flexible(child: ClideText(widget.entry.message, fontSize: clideFontBody, maxLines: 3, overflow: TextOverflow.ellipsis)),
+                  Flexible(
+                    child: ClideText(widget.entry.message, fontSize: clideFontBody, maxLines: 3, overflow: TextOverflow.ellipsis),
+                  ),
                   const SizedBox(width: clideGapStandard),
                   Semantics(
                     button: true,
                     label: 'Dismiss notification',
                     child: ClideTappable(
                       onTap: widget.onDismiss,
-                      builder: (ctx, hovered, _) => ClideIcon(
-                        PhosphorIcons.byName('x'),
-                        size: clideIconStandard,
-                        color: hovered ? tokens.globalForeground : tokens.globalTextMuted,
-                      ),
+                      builder: (ctx, hovered, _) =>
+                          ClideIcon(PhosphorIcons.byName('x'), size: clideIconStandard, color: hovered ? tokens.globalForeground : tokens.globalTextMuted),
                     ),
                   ),
                 ],