dashboard: show macOS version in debug mode (#1454)

## Motivation

When debugging cluster issues, it's useful to see which macOS version
each node is running — especially since version mismatches can cause
compatibility problems. The OS version data is already collected by the
identity gatherer but wasn't shown in the topology graph.

## Changes

- Added OS version label (e.g. "macOS 15.2") to each node in the
topology graph when debug mode is enabled
- Renders below the existing TB and RDMA debug labels using the same
styling conventions
- Sources data from the existing `nodeIdentities` store (no backend
changes needed)

## Why It Works

The `nodeIdentities` store already contains `osVersion` for each node.
We simply read it in the `TopologyGraph` component and append a text
label in the debug section, following the exact same pattern as the TB
and RDMA labels.

## Test Plan

### Manual Testing
<!-- Hardware: MacBook Pro -->
- Enable debug mode in the dashboard
- Verify OS version label appears below TB/RDMA labels on each node
- Verify label disappears when debug mode is disabled

### Automated Testing
- Dashboard build passes (`npm run build`)

---------

Co-authored-by: Claude Opus 4.6 <[email protected]>
Co-authored-by: rltakashige <[email protected]>
Co-authored-by: Ryuichi Leo Takashige <[email protected]>

dashboard/src/lib/components/TopologyGraph.svelte

@@ -7,6 +7,7 @@
     debugMode,
     nodeThunderboltBridge,
     nodeRdmaCtl,
+    nodeIdentities,
     type NodeInfo,
   } from "$lib/stores/app.svelte";
 
@@ -33,6 +34,7 @@
   const debugEnabled = $derived(debugMode());
   const tbBridgeData = $derived(nodeThunderboltBridge());
   const rdmaCtlData = $derived(nodeRdmaCtl());
+  const identitiesData = $derived(nodeIdentities());
 
   function getNodeLabel(nodeId: string): string {
     const node = data?.nodes?.[nodeId];
@@ -1177,6 +1179,22 @@
             .attr("font-size", debugFontSize)
             .attr("font-family", "SF Mono, Monaco, monospace")
             .text(rdmaText);
+          debugLabelY += debugLineHeight;
+        }
+
+        const identity = identitiesData[nodeInfo.id];
+        if (identity?.osVersion) {
+          nodeG
+            .append("text")
+            .attr("x", nodeInfo.x)
+            .attr("y", debugLabelY)
+            .attr("text-anchor", "middle")
+            .attr("fill", "rgba(179,179,179,0.7)")
+            .attr("font-size", debugFontSize)
+            .attr("font-family", "SF Mono, Monaco, monospace")
+            .text(
+              `macOS ${identity.osVersion}${identity.osBuildVersion ? ` (${identity.osBuildVersion})` : ""}`,
+            );
         }
       }
     });