Linux: fix taskbar pin matching and launcher icon mismatch

Add StartupWMClass=OrbitHub to the desktop file so panels (GNOME
Shell, Cinnamon, KDE) can match the running window to the pinned
launcher instead of creating a duplicate taskbar entry.

Replace the hand-authored, stale launcher SVG with PNG icons
rendered directly from createOrbitHubAppIcon() at each hicolor
theme size, so the pinned/menu icon matches the actual app icon
shown in the running window, Windows .ico, and macOS .icns.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

CMakeLists.txt

@@ -232,10 +232,12 @@ install(FILES
     packaging/linux/org.darksingularity.OrbitHub.desktop
     DESTINATION ${CMAKE_INSTALL_DATADIR}/applications
 )
-install(FILES
-    packaging/linux/org.darksingularity.OrbitHub.svg
-    DESTINATION ${CMAKE_INSTALL_DATADIR}/icons/hicolor/scalable/apps
-)
+foreach(icon_size IN ITEMS 16 24 32 48 64 128 256)
+    install(FILES
+        packaging/linux/icons/${icon_size}x${icon_size}/apps/org.darksingularity.OrbitHub.png
+        DESTINATION ${CMAKE_INSTALL_DATADIR}/icons/hicolor/${icon_size}x${icon_size}/apps
+    )
+endforeach()
 install(FILES
     packaging/linux/org.darksingularity.OrbitHub.metainfo.xml
     DESTINATION ${CMAKE_INSTALL_DATADIR}/metainfo