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.tsx

@@ -94,6 +94,7 @@ export default function EditorLayout() {
     pinned,
     isCollapsed, toggleCollapse,
     remoteSearchLoading,
+    remoteSearchFailedNodes,
   } = stackListState;
 
   const { nodes, activeNode, setActiveNode } = useNodes();
@@ -390,6 +391,7 @@ export default function EditorLayout() {
           buildMenuCtx,
           remoteResults,
           remoteLoading: remoteSearchLoading,
+          remoteFailedNodes: remoteSearchFailedNodes,
           onSelectRemoteFile: (nodeId, file) => {
             const node = nodes.find(n => n.id === nodeId);
             if (node) void stackActions.loadFileOnNode(node, file);

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;
 }

frontend/src/components/sidebar/StackList.tsx

@@ -1,5 +1,5 @@
-import { useMemo } from 'react';
-import { ArrowUpRight, Loader2 } from 'lucide-react';
+import { useMemo, useState } from 'react';
+import { ArrowUpRight, Loader2, AlertCircle, ChevronDown, ChevronRight } from 'lucide-react';
 import { useStackKeyboardShortcuts } from '@/hooks/useStackKeyboardShortcuts';
 import { CommandItem, CommandList } from '@/components/ui/command';
 import { Skeleton } from '@/components/ui/skeleton';
@@ -18,6 +18,12 @@ interface RemoteNodeResult {
   files: { file: string; status: StackRowStatus }[];
 }
 
+interface RemoteSearchFailure {
+  nodeId: number;
+  nodeName: string;
+  reason: string;
+}
+
 export interface StackListProps {
   files: string[];
   isLoading: boolean;
@@ -37,6 +43,7 @@ export interface StackListProps {
   buildMenuCtx: (file: string) => StackMenuCtx;
   remoteResults: RemoteNodeResult[];
   remoteLoading: boolean;
+  remoteFailedNodes: RemoteSearchFailure[];
   onSelectRemoteFile: (nodeId: number, file: string) => void;
 }
 
@@ -107,9 +114,11 @@ export function StackList(props: StackListProps & StackListBulkProps) {
     stackUpdates, gitSourcePendingMap, pinnedFiles, isCollapsed, toggleCollapse,
     isBusy, getDisplayName, onSelectFile, buildMenuCtx,
     bulkMode, selectedFiles, onToggleSelect,
-    remoteResults, remoteLoading, onSelectRemoteFile,
+    remoteResults, remoteLoading, remoteFailedNodes, onSelectRemoteFile,
   } = props;
 
+  const [failedNodesExpanded, setFailedNodesExpanded] = useState(false);
+
   const groups = useMemo(
     () => buildGroups(files, pinnedFiles, stackLabelMap),
     [files, pinnedFiles, stackLabelMap],
@@ -171,12 +180,39 @@ export function StackList(props: StackListProps & StackListBulkProps) {
         </StackGroup>
       ))}
 
-      {searchQuery.trim() && (remoteLoading || remoteResults.length > 0) && (
+      {searchQuery.trim() && (remoteLoading || remoteResults.length > 0 || remoteFailedNodes.length > 0) && (
         <div className="mt-3 pt-3 border-t border-glass-border">
           <h3 className="text-[10px] font-medium tracking-[0.08em] uppercase text-stat-subtitle px-4 pb-2 flex items-center gap-2">
             Other nodes
             {remoteLoading && <Loader2 className="w-3 h-3 animate-spin text-stat-icon" strokeWidth={1.5} />}
           </h3>
+          {remoteFailedNodes.length > 0 && (
+            <button
+              type="button"
+              onClick={() => setFailedNodesExpanded(prev => !prev)}
+              className="w-full mx-4 mb-2 flex items-center gap-1.5 text-[10px] font-mono uppercase tracking-[0.06em] text-warning hover:text-warning/80 cursor-pointer"
+              style={{ width: 'calc(100% - 2rem)' }}
+              aria-expanded={failedNodesExpanded}
+              title={remoteFailedNodes.map(f => `${f.nodeName}: ${f.reason}`).join('\n')}
+            >
+              <AlertCircle className="w-3 h-3 shrink-0" strokeWidth={1.5} />
+              <span className="shrink-0">
+                {remoteFailedNodes.length} {remoteFailedNodes.length === 1 ? 'node' : 'nodes'} unreachable
+              </span>
+              {failedNodesExpanded
+                ? <ChevronDown className="w-3 h-3 shrink-0" strokeWidth={1.5} />
+                : <ChevronRight className="w-3 h-3 shrink-0" strokeWidth={1.5} />}
+            </button>
+          )}
+          {failedNodesExpanded && remoteFailedNodes.length > 0 && (
+            <div className="px-4 pb-2 space-y-0.5">
+              {remoteFailedNodes.map(f => (
+                <div key={f.nodeId} className="text-[10px] font-mono text-stat-subtitle truncate">
+                  <span className="text-warning">·</span> {f.nodeName}: {f.reason}
+                </div>
+              ))}
+            </div>
+          )}
           {remoteResults.map(({ nodeId, nodeName, files: remoteFiles }) => (
             <div key={nodeId} className="mb-2">
               <div className="px-4 pb-1 flex items-center gap-1.5 text-[10px] font-mono uppercase tracking-[0.06em] text-stat-subtitle">