fix(library): a listing poll can no longer put back an older resume point

A poll already out when the player closed answered with the point the server held before the
closing beacon, over the row's patch; Continue then opened there and wrote it back. The close now
cancels the listings' fetches before patching and reads them again once the beacon has landed.

frontend/src/lib/api/core.ts

@@ -288,8 +288,9 @@ export function identityHeaders(): Record<string, string> {
 // Fire-and-forget POST that survives page unload (keepalive) — shared by the resume-position
 // beacons (YT + Plex players). A normal fetch is cancelled on unload and React effect cleanup
 // doesn't run on a full reload, so without keepalive the last position (esp. right after a seek)
-// would be lost.
-export function beacon(url: string, body: Record<string, unknown>): void {
+// would be lost. `onSettled`: the request has been answered (or failed) — for a caller whose cached
+// view must be read again once the write is on the server.
+export function beacon(url: string, body: Record<string, unknown>, onSettled?: () => void): void {
   try {
     void send(url, {
       method: "POST",
@@ -297,7 +298,10 @@ export function beacon(url: string, body: Record<string, unknown>): void {
       credentials: "include",
       headers: { "Content-Type": "application/json", ...identityHeaders() },
       body: JSON.stringify(body),
-    }).catch(() => {});
+    }).then(
+      () => onSettled?.(),
+      () => onSettled?.(),
+    );
   } catch {
     /* ignore */
   }

frontend/src/lib/api/downloads.ts

@@ -299,11 +299,16 @@ export const downloadsApi = {
     id: number,
     positionSeconds: number,
     durationSeconds: number,
+    onSettled?: () => void,
   ): void =>
-    beacon(`/api/downloads/${id}/progress`, {
-      position_seconds: Math.floor(positionSeconds),
-      duration_seconds: Math.floor(durationSeconds),
-    }),
+    beacon(
+      `/api/downloads/${id}/progress`,
+      {
+        position_seconds: Math.floor(positionSeconds),
+        duration_seconds: Math.floor(durationSeconds),
+      },
+      onSettled,
+    ),
   // A plain <a> navigation can't send the X-Siftlode-Account header, so pass the active
   // account via ?account= (the server honours it only for an account in this browser's wallet) —
   // otherwise the download resolves to the session-default account and 404s for a per-tab