Keep the knowledge graph when one schedule fails to load

One failing per-schedule request no longer blanks the whole graph, and
the graph shows the engine's own error instead of always saying the
engine cannot be reached.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/pages/Production.tsx

@@ -92,7 +92,8 @@ export function ProductionPage() {
         </button>
         {graph
           ? <KnowledgeGraph graph={graph} />
-          : <div className="graph-loading" role="status">{docs.isError || results.isError ? t('brain.engineDown') : t('prod.loadingGraph')}</div>}
+          // The engine's own message: an unreachable engine says so, any other failure says what it was.
+          : <div className="graph-loading" role="status">{(docs.error ?? results.error)?.message ?? t('prod.loadingGraph')}</div>}
 
         <aside className="graph-panel">
           <div className="graph-stats">