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/kernel/src/i18n/fallback_chain.dart

@@ -14,22 +14,14 @@ import 'package:flutter/foundation.dart';
 /// canonicalized by omitting it (not empty string) so equality works.
 @immutable
 class FallbackChain {
-  const FallbackChain({
-    required this.current,
-    required this.defaultLocale,
-  });
+  const FallbackChain({required this.current, required this.defaultLocale});
 
   final Locale current;
   final Locale defaultLocale;
 
   List<Locale> resolve() {
     final out = <Locale>[];
-    for (final l in [
-      current,
-      Locale(current.languageCode),
-      defaultLocale,
-      Locale(defaultLocale.languageCode),
-    ]) {
+    for (final l in [current, Locale(current.languageCode), defaultLocale, Locale(defaultLocale.languageCode)]) {
       final canon = _canon(l);
       if (!out.any((e) => _canon(e) == canon)) {
         out.add(l);