add "list" to step expansion modes

When you select LIST mode:
applyModeSteps("list", showUtils) is called
- Groups expand because: mode !== "collapsed" → "list" !== "collapsed" ✓
- Steps stay collapsed because: mode === "expanded" → "list" === "expanded" ✗
- During streaming, steps don't auto-expand because the condition checks for detailMode === "current", which is false.

This is why we only need one line for this new modality to work.

webui/components/sidebar/bottom/preferences/preferences-store.js

@@ -73,6 +73,7 @@ const model = {
   // Detail mode options for UI sidebar
   detailModeOptions: [
     { label: "NONE", value: "collapsed", title: "All collapsed" },
+    { label: "LIST", value: "list", title: "Steps collapsed" },
     { label: "STEP", value: "current", title: "Current step only" },
     { label: "ALL", value: "expanded", title: "All expanded" },
   ],