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/extension/src/extension.dart

@@ -98,15 +98,6 @@ extension ClideExtensionContextI18n on ClideExtensionContext {
   String t(String key, {String? placeholder}) => i18n.string(key, namespace: id, placeholder: placeholder);
 
   /// [t] with interpolation replacers.
-  String tr(
-    String key, {
-    String? placeholder,
-    List<I18nReplacer> replacers = const [],
-  }) =>
-      i18n.interpolated(
-        key,
-        namespace: id,
-        placeholder: placeholder,
-        replacers: replacers,
-      );
+  String tr(String key, {String? placeholder, List<I18nReplacer> replacers = const []}) =>
+      i18n.interpolated(key, namespace: id, placeholder: placeholder, replacers: replacers);
 }