fix(deferred): address /code-review findings on the closeout diff

- channels CB3: sort discovery rows NULLs-last (title is None) to match the DB
  ORDER BY exactly — coercing NULL title to "" floated untitled channels to the top.
- AdminUsers SC1: track in-flight rows in a Set, not a single id — per-row disabling
  now lets the admin start concurrent row actions, and a shared id was cleared by
  whichever settled first (re-enabling a still-pending row → possible double-submit).
- ChatThread CT4: reset the incoming-count ref when partnerId changes — the Messages
  page reuses one ChatThread across conversation switches, so a same-count switch
  could skip the open-marks-read badge refresh.
- messages MB3: push a bare {type:"unread"} (drop the now-dead count query — the
  client re-fetches on the signal and ignored the number anyway).
- api.ts: extract the shared keepalive `beacon()` helper (saveProgressBeacon +
  plexProgressBeacon were near-verbatim copies).

frontend/src/lib/api.ts

@@ -299,6 +299,29 @@ const setupHeaders = (token: string) => ({
 const ACTIVE_ACCOUNT_HEADER = "X-Siftlode-Account";
 
 export const getActiveAccount = activeAccountId;
+
+// 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.
+function beacon(url: string, body: Record<string, unknown>): void {
+  const active = getActiveAccount();
+  try {
+    void fetch(url, {
+      method: "POST",
+      keepalive: true,
+      credentials: "include",
+      headers: {
+        "Content-Type": "application/json",
+        ...(active != null ? { [ACTIVE_ACCOUNT_HEADER]: String(active) } : {}),
+      },
+      body: JSON.stringify(body),
+    }).catch(() => {});
+  } catch {
+    /* ignore */
+  }
+}
+
 export function setActiveAccount(id: number): void {
   try {
     sessionStorage.setItem(ACTIVE_ACCOUNT_KEY, String(id));
@@ -1102,30 +1125,12 @@ export const api = {
       },
       { idempotent: true }
     ),
-  // Fire-and-forget YT progress save that survives page unload (F5/close/navigate) — mirrors
-  // plexProgressBeacon. A normal fetch is cancelled on unload and React effect cleanup doesn't run
-  // on a full reload, so the last position (esp. right after a seek) would be lost; keepalive lets
-  // it complete.
-  saveProgressBeacon: (id: string, positionSeconds: number, durationSeconds: number): void => {
-    const active = getActiveAccount();
-    try {
-      void fetch(`/api/videos/${encodeURIComponent(id)}/progress`, {
-        method: "POST",
-        keepalive: true,
-        credentials: "include",
-        headers: {
-          "Content-Type": "application/json",
-          ...(active != null ? { [ACTIVE_ACCOUNT_HEADER]: String(active) } : {}),
-        },
-        body: JSON.stringify({
-          position_seconds: Math.floor(positionSeconds),
-          duration_seconds: Math.floor(durationSeconds),
-        }),
-      }).catch(() => {});
-    } catch {
-      /* ignore */
-    }
-  },
+  // Fire-and-forget YT progress save that survives page unload (F5/close/navigate).
+  saveProgressBeacon: (id: string, positionSeconds: number, durationSeconds: number): void =>
+    beacon(`/api/videos/${encodeURIComponent(id)}/progress`, {
+      position_seconds: Math.floor(positionSeconds),
+      duration_seconds: Math.floor(durationSeconds),
+    }),
   videoDetail: (id: string): Promise<VideoDetail> => req(`/api/videos/${id}`),
   pauseSync: () => req("/api/sync/pause", { method: "POST" }),
   resumeSync: () => req("/api/sync/resume", { method: "POST" }),
@@ -1358,23 +1363,12 @@ export const api = {
     position_seconds: number,
     duration_seconds: number,
     final = false,
-  ): void => {
-    const active = getActiveAccount();
-    try {
-      void fetch(`/api/plex/item/${encodeURIComponent(id)}/progress`, {
-        method: "POST",
-        keepalive: true,
-        credentials: "include",
-        headers: {
-          "Content-Type": "application/json",
-          ...(active != null ? { [ACTIVE_ACCOUNT_HEADER]: String(active) } : {}),
-        },
-        body: JSON.stringify({ position_seconds, duration_seconds, final }),
-      }).catch(() => {});
-    } catch {
-      /* ignore */
-    }
-  },
+  ): void =>
+    beacon(`/api/plex/item/${encodeURIComponent(id)}/progress`, {
+      position_seconds,
+      duration_seconds,
+      final,
+    }),
   plexSetState: (id: string, status: "new" | "watched" | "hidden"): Promise<unknown> =>
     req(`/api/plex/item/${encodeURIComponent(id)}/state`, {
       method: "POST",