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>

test/editor/registry_test.dart

@@ -138,10 +138,7 @@ void main() {
 
   test('contentFromArgs decodes content_b64 when text is absent', () {
     expect(EditorRegistry.contentFromArgs({'text': 'plain'}), 'plain');
-    expect(
-      EditorRegistry.contentFromArgs({'content_b64': 'aGVsbG8='}),
-      'hello',
-    );
+    expect(EditorRegistry.contentFromArgs({'content_b64': 'aGVsbG8='}), 'hello');
     expect(EditorRegistry.contentFromArgs(const {}), '');
   });