Fix file names to respect conventions

Important markdown/text files: UPPERCASE.ext
QML module dirs, qml files: PascalCase
Python module dirs, py files, js files: snake_case
Everything else (including icons): hyphen-case

src/qml/UI.qml

@@ -25,7 +25,7 @@ Item {
     HImage {
         id: mainUIBackground
         fillMode: Image.PreserveAspectCrop
-        source: "../images/login_background.jpg"
+        source: "../images/background.jpg"
         sourceSize.width: Screen.width
         sourceSize.height: Screen.height
         anchors.fill: parent