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_pty_view.dart

@@ -14,14 +14,7 @@ import 'package:clide/src/terminal/terminal.dart';
 /// `terminal.write()` is the consumer's job (typically a builtin
 /// extension — see `builtin.terminal` / `builtin.claude`).
 class ClidePtyView extends StatelessWidget {
-  const ClidePtyView({
-    super.key,
-    required this.terminal,
-    this.label,
-    this.focusNode,
-    this.autofocus = false,
-    this.fontSize = clideFontMono,
-  });
+  const ClidePtyView({super.key, required this.terminal, this.label, this.focusNode, this.autofocus = false, this.fontSize = clideFontMono});
 
   final Terminal terminal;
 
@@ -49,11 +42,7 @@ class ClidePtyView extends StatelessWidget {
           focusNode: focusNode,
           autofocus: autofocus,
           theme: _buildTheme(tokens),
-          textStyle: TerminalStyle(
-            fontSize: fontSize,
-            fontFamily: clideMonoFamily,
-            fontFamilyFallback: clideMonoFamilyFallback,
-          ),
+          textStyle: TerminalStyle(fontSize: fontSize, fontFamily: clideMonoFamily, fontFamilyFallback: clideMonoFamilyFallback),
           padding: const EdgeInsets.all(2),
           backgroundOpacity: 1,
           cursorType: TerminalCursorType.block,