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_button.dart

@@ -49,8 +49,8 @@ class ClideButton extends StatelessWidget {
               bg = pressed
                   ? tokens.buttonActiveBackground
                   : hovered
-                      ? tokens.buttonHoverBackground
-                      : tokens.buttonBackground;
+                  ? tokens.buttonHoverBackground
+                  : tokens.buttonBackground;
               fg = tokens.buttonForeground;
             case ClideButtonVariant.primary:
               bg = pressed ? tokens.panelActiveBorder : tokens.buttonActiveBackground;
@@ -67,11 +67,7 @@ class ClideButton extends StatelessWidget {
               border: Border.all(color: tokens.buttonBorder, width: 1),
               borderRadius: BorderRadius.circular(3),
             ),
-            child: ClideText(
-              label,
-              color: fg,
-              fontWeight: variant == ClideButtonVariant.primary ? FontWeight.w600 : FontWeight.w500,
-            ),
+            child: ClideText(label, color: fg, fontWeight: variant == ClideButtonVariant.primary ? FontWeight.w600 : FontWeight.w500),
           );
         },
       ),