fix(ui): respect Plasma style and add app branding

- Preserve the platform-selected QtQuick style on Plasma
- Add the QtPotato desktop entry and bundled app icon
- Remove the empty-state open-file action

src/qml/MpvVideo.qml

@@ -67,7 +67,6 @@ MpvItem {
         icon.name: "video-symbolic"
         text: KI18n.i18nc("@info:placeholder", "No video open")
         explanation: KI18n.i18nc("@info:placeholder", "Open a file or drop one here")
-        helpfulAction: root.actions.openFile
     }
 
     Timer {