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.

tests/download-manager.test.ts

@@ -45,6 +45,44 @@ describe("runWithLimitedConcurrency", () => {
   });
 });
 
+describe("download live update cadence", () => {
+  it.each([69, 661, 2_470])("emits a running queue snapshot no sooner than 750 ms for %i items", async (itemCount) => {
+    vi.useFakeTimers();
+    try {
+      const root = fs.mkdtempSync(path.join(os.tmpdir(), "rd-live-cadence-"));
+      tempDirs.push(root);
+      const session = emptySession();
+      session.running = true;
+      const manager = new DownloadManager(defaultSettings(), session, createStoragePaths(path.join(root, "state")));
+      const internal = manager as unknown as {
+        emitState: () => void;
+        itemCount: number;
+        session: typeof session;
+        stateEmitTimer: NodeJS.Timeout | null;
+      };
+      internal.itemCount = itemCount;
+      internal.session.running = true;
+      if (internal.stateEmitTimer) {
+        clearTimeout(internal.stateEmitTimer);
+        internal.stateEmitTimer = null;
+      }
+      let emitted = 0;
+      manager.on("state", () => {
+        emitted += 1;
+      });
+
+      internal.emitState();
+      await vi.advanceTimersByTimeAsync(749);
+      expect(emitted).toBe(0);
+
+      await vi.advanceTimersByTimeAsync(1);
+      expect(emitted).toBe(1);
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+});
+
 describe("resolveUnrestrictTimeoutBudgetMs", () => {
   it("covers the complete provider plan and each later Real-Debrid account attempt", () => {
     const settings = {