feat(sidebar): surface unreachable nodes in cross-node stack search (#1195)

Per-node fetch failures in useCrossNodeStackSearch were silently
swallowed into []. The user could not tell the difference between
"this node has no matching stack" and "this node timed out or 502'd".

Adds a third return value to the hook: failedNodes: FailedNode[]
where FailedNode is { nodeId, nodeName, reason }. The reason captures
the HTTP status text (e.g. "list returned HTTP 502") or the
underlying Error message ("connect ECONNREFUSED 192.168.x.x:1852")
without leaking node URLs. AbortError from the effect cleanup path
is intentionally excluded - it's expected when the user keeps typing.

Threads the new field through useStackListState and EditorLayout into
StackList. StackList renders a warning chip below the "Other nodes"
header when the array is non-empty:

  ! N nodes unreachable    > (expand)

Click expands the chip to a vertical list of "node: reason" lines.
Hover surfaces the full reasons in the title attribute too, so a
user inspecting at a glance can read them without clicking. The chip
is suppressed entirely when the search yields no failures (no zero
state). Color is the existing warning token, not error - the failure
is recoverable (node may come back) and not a system-wide problem.

GlobalCommandPalette also calls useCrossNodeStackSearch but ignores
the third return value; its destructuring is additive-safe.

Resolves M-2 from the stack-management audit.

frontend/src/components/EditorLayout/hooks/useStackListState.ts

@@ -54,7 +54,7 @@ export function useStackListState() {
   const { isCollapsed, toggle: toggleCollapse } = useSidebarGroupCollapse(activeNode?.id);
   const { runBulk } = useBulkStackActions();
 
-  const { hits: remoteSearchHits, loading: remoteSearchLoading } = useCrossNodeStackSearch({
+  const { hits: remoteSearchHits, failedNodes: remoteSearchFailedNodes, loading: remoteSearchLoading } = useCrossNodeStackSearch({
     query: searchQuery,
     enabled: true,
     excludeNodeId: activeNode?.id,
@@ -359,5 +359,6 @@ export function useStackListState() {
     pinned, pin, unpin, isPinned,
     isCollapsed, toggleCollapse,
     remoteSearchLoading,
+    remoteSearchFailedNodes,
   } as const;
 }