perf(ui): enforce 750 ms live update cadence

Use one 750 ms interval for running download snapshots, progress updates, the bandwidth chart, and the header speed sparkline across every queue size. Remove the extra active-download renderer delay and add timing regressions for small, medium, and large queues.

src/renderer/App.tsx

@@ -880,7 +880,7 @@ const historyRetentionLabels: Record<RendererSettings["historyRetentionMode"], s
 const AUTO_RENDER_PACKAGE_LIMIT = 260;
 
 export function getSnapshotRenderDelay(itemCount: number, running: boolean, activeTab: MainView): number {
-  let delay = itemCount >= 250 ? 0 : 100;
+  let delay = running && activeTab === "downloads" ? 0 : itemCount >= 250 ? 0 : 100;
   if (!running) delay = Math.min(delay, 200);
   if (activeTab !== "downloads") delay = Math.max(delay, 800);
   return delay;
@@ -1272,7 +1272,7 @@ const BandwidthChart = memo(function BandwidthChart({ running, paused, speedHist
     const reducedMotion = window.matchMedia("(prefers-reduced-motion: reduce)").matches;
     const interval = setInterval(() => {
       drawChart();
-    }, reducedMotion ? 1000 : 500);
+    }, reducedMotion ? 1000 : 750);
     return () => clearInterval(interval);
   }, [drawChart, running, paused]);
 
@@ -1374,7 +1374,7 @@ const DownloadSpeedSparkline = memo(function DownloadSpeedSparkline({ speedBps,
       draw();
     };
 
-    const id = window.setInterval(tick, 500);
+    const id = window.setInterval(tick, 750);
     return () => window.clearInterval(id);
   }, []);