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_accordion.dart

@@ -39,8 +39,12 @@ class ClideAccordion extends StatelessWidget {
                 ClideIcon(expanded ? PhosphorIcons.byName('caret-down') : PhosphorIcons.byName('caret-right'), size: 10, color: tokens.globalTextMuted),
                 const SizedBox(width: 6),
                 if (leading != null) ...[leading!, const SizedBox(width: 6)],
-                ClideText('$label · $count',
-                    fontSize: clideFontSmall, color: hovered ? tokens.globalForeground : tokens.sidebarSectionHeader, fontFamily: clideMonoFamily),
+                ClideText(
+                  '$label · $count',
+                  fontSize: clideFontSmall,
+                  color: hovered ? tokens.globalForeground : tokens.sidebarSectionHeader,
+                  fontFamily: clideMonoFamily,
+                ),
               ],
             ),
           ),