Fix sparklines not rendering on initial page load in Chromium

Replace useId() with colon-free ref-based ID generator to avoid SVG
url() gradient resolution failures, and add placeholderData to
timeseries query to prevent flash during refetch.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/queries/executions.ts

@@ -46,6 +46,7 @@ export function useStatsTimeseries(timeFrom: string | undefined, timeTo: string
       return data!;
     },
     enabled: !!timeFrom,
+    placeholderData: (prev) => prev,
     refetchInterval: 30_000,
   });
 }