fix(windows): require Flutter 3.44 for pen input

Surface Pen reports as PointerDeviceKind.stylus and multitouch is
delivered only on Flutter 3.44+ (WM_POINTER migration, PR #165323).
3.41.4 dropped pen events, so our stylus-kind capture got nothing and
pinch-zoom did not work. Pin CI to Flutter 3.44.2 (shadowing any cached
older SDK on the runner) and require flutter >=3.44.0 in pubspec.

pubspec.yaml

@@ -5,6 +5,10 @@ version: 0.1.0+1
 
 environment:
   sdk: ^3.10.8
+  # pdfrx + Surface Pen require Windows stylus/multitouch, which only exists
+  # in Flutter 3.44+ (WM_POINTER migration, PR #165323). Building on older
+  # Flutter silently breaks pen input on Windows.
+  flutter: ">=3.44.0"
 
 dependencies:
   flutter: