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/builtin/vim/src/vim_mode_indicator.dart

@@ -20,12 +20,7 @@ class VimModeIndicator extends StatelessWidget {
         final tokens = ClideTheme.of(context).surface;
         return Padding(
           padding: const EdgeInsets.symmetric(horizontal: 8),
-          child: ClideText(
-            '-- ${service.mode.label} --',
-            fontFamily: clideMonoFamily,
-            fontSize: clideFontCaption,
-            color: tokens.statusBarForeground,
-          ),
+          child: ClideText('-- ${service.mode.label} --', fontFamily: clideMonoFamily, fontSize: clideFontCaption, color: tokens.statusBarForeground),
         );
       },
     );