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_file_image.dart

@@ -13,9 +13,7 @@ import 'package:flutter/widgets.dart';
 /// into `==`/`hashCode` makes an overwrite a fresh key → a cache miss → a
 /// re-read of the current bytes; an unchanged file still hits the cache.
 class ClideFileImage extends FileImage {
-  ClideFileImage(String path, {double scale = 1.0})
-      : _stamp = _stampOf(path),
-        super(File(path), scale: scale);
+  ClideFileImage(String path, {double scale = 1.0}) : _stamp = _stampOf(path), super(File(path), scale: scale);
 
   /// mtime ⊕ size — changes on any in-place overwrite (a write bumps mtime; a
   /// different length bumps size even within one clock tick). 0 if the file