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/icons/phosphor.dart

@@ -11,9 +11,7 @@ class PhosphorIconPainter extends ClideIconPainter {
 
   @override
   void paint(ui.Canvas canvas, ui.Color color) {
-    final builder = ui.ParagraphBuilder(
-      ui.ParagraphStyle(fontFamily: family, fontSize: 1.0, height: 1.0, textAlign: ui.TextAlign.center),
-    )
+    final builder = ui.ParagraphBuilder(ui.ParagraphStyle(fontFamily: family, fontSize: 1.0, height: 1.0, textAlign: ui.TextAlign.center))
       ..pushStyle(ui.TextStyle(color: color, fontFamily: family))
       ..addText(String.fromCharCode(codePoint));
     final paragraph = builder.build()..layout(const ui.ParagraphConstraints(width: 1.0));