ci: ship Windows .ico icon for PyInstaller build

PyInstaller on Windows requires an .ico (or .exe) for the executable icon and
its PNG->ICO auto-conversion did not engage on the runner. Add a multi-size
Macro Pad.ico (16-256px) generated from Macro Pad.png and point macropad.spec
at it. The PNG stays bundled for the runtime window/tray icon.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

macropad.spec

@@ -81,5 +81,5 @@ exe = EXE(
     target_arch=None,
     codesign_identity=None,
     entitlements_file=None,
-    icon='Macro Pad.png',
+    icon='Macro Pad.ico',
 )