feat: sync pane workspace overhaul from private repo

- ship the expanded left-nav and pane header workspace updates
- add dimension-driven browse flow, map view upgrades, and delete-node tooling
- expand eval coverage and refresh the public UI docs for the new layout

Generated with Codex

app/api/dimensions/route.ts

@@ -59,7 +59,9 @@ export async function POST(request: NextRequest) {
       }, { status: 400 });
     }
 
-    const descriptionError = validateDimensionDescription(description || '');
+    const descriptionError = description !== null
+      ? validateDimensionDescription(description)
+      : null;
     if (descriptionError) {
       return NextResponse.json({
         success: false,