Android launcher: drop monochrome (themed) icon, keep color only

The monochrome adaptive-icon layer produced a poor themed-icon rendering.
Remove the <monochrome> reference from mipmap-anydpi-v26/ic_launcher.xml and
delete the ic_launcher_monochrome.png files so Android always uses the color
adaptive icon (background + foreground). sync-android-sources.sh also drops any
monochrome layer Tauri regenerates.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/android/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -2,5 +2,4 @@
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
   <background android:drawable="@mipmap/ic_launcher_background"/>
   <foreground android:drawable="@mipmap/ic_launcher_foreground"/>
-  <monochrome android:drawable="@mipmap/ic_launcher_monochrome"/>
 </adaptive-icon>