missions: no-store fetch + visible updated-at timestamp on refresh

- api client: cache: no-store so manual Refresh guarantees a fresh
  server response (was potentially hitting stale HTTP cache).
- MissionCanvas: renders "updated HH:MM:SS" next to the refresh
  button; the timestamp bumps on every successful load so the click
  is visibly acknowledged even when nothing else on the page changed.

frontend/src/components/dashboard/MissionCanvas.tsx

@@ -118,6 +118,7 @@ export function MissionCanvas({
   const [editOpen, setEditOpen] = useState(false);
   const [deleteBusy, setDeleteBusy] = useState(false);
   const [runs, setRuns] = useState<MissionRunSummary[]>([]);
+  const [lastLoadedAt, setLastLoadedAt] = useState<Date | null>(null);
 
   const load = useCallback(async () => {
     if (!selectedId) {
@@ -133,6 +134,7 @@ export function MissionCanvas({
       ]);
       setMission(m);
       setRuns(r.runs);
+      setLastLoadedAt(new Date());
     } catch (e) {
       setError(e instanceof Error ? e.message : "load failed");
     } finally {
@@ -424,6 +426,18 @@ export function MissionCanvas({
                 {refining ? "Refining…" : "Refine"}
               </button>
             )}
+            {lastLoadedAt && (
+              <span
+                style={{
+                  fontSize: 10,
+                  color: "#6a6a72",
+                  fontFamily: mono,
+                }}
+                title={lastLoadedAt.toISOString()}
+              >
+                updated {lastLoadedAt.toLocaleTimeString()}
+              </span>
+            )}
             <button
               type="button"
               onClick={load}