fix dll overrides & add disable UMU

src/library.js

@@ -217,6 +217,9 @@ function newGame({ id, name, exePath, workdir, mode }) {
     prefix: '',
     args: '',
     dllOverrides: [],
+    // Proton-only switch: keep UMU (the default path) or fall back to the
+    // build's steam.exe launcher for this app (see runner.buildEnv)
+    disableUmu: false,
     env: {},
     iconSource: '',
     playSeconds: 0,