fix(ui): R3 review — give the Stats API-usage block its own loading/error state

The 'usage' query rendered both its loading and its error state as 'No usage' (the same false-empty
R3 targets), while the sibling 'status' query got a proper state. Split the fallback into
error(+retry) / loading / empty.

frontend/src/components/Stats.tsx

@@ -155,6 +155,12 @@ function Overview({ me }: { me: Me }) {
               </div>
             )}
           </>
+        ) : usage.isError ? (
+          <StateMessage tone="error" onRetry={() => usage.refetch()}>
+            {t("common.loadError")}
+          </StateMessage>
+        ) : usage.isLoading ? (
+          <LoadingState label={t("stats.sync.loading")} />
         ) : (
           <div className="text-muted text-sm">{t("stats.sync.noUsage")}</div>
         )}