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/daemon/git_commands_errors_test.dart

@@ -40,8 +40,8 @@ void main() {
     (
       'git.stage',
       const {
-        'paths': ['file.txt']
-      }
+        'paths': ['file.txt'],
+      },
     ),
     ('git.stage-all', const <String, Object?>{}),
     ('git.unstage', const <String, Object?>{}),
@@ -53,8 +53,8 @@ void main() {
     (
       'git.discard',
       const {
-        'paths': ['file.txt']
-      }
+        'paths': ['file.txt'],
+      },
     ),
     ('git.commit', const {'message': 'hi'}),
     ('git.stash', const <String, Object?>{}),