fix: icon display in application menu

- Changed Icon path to absolute in desktop file
- Fixed icon file permissions (chmod 644)
- Icon now shows correctly in GNOME/KDE menus

Desktop file updated: Icon=umbrix → Icon=/usr/share/icons/hicolor/256x256/apps/umbrix.png

linux/app_icon.desktop

@@ -2,7 +2,7 @@
 Name=Umbrix
 Comment=Cross Platform Multi Protocol Proxy Frontend
 Exec=umbrix %U
-Icon=umbrix
+Icon=/usr/share/icons/hicolor/256x256/apps/umbrix.png
 Terminal=false
 Type=Application
 Categories=Network;