fix(header): show per-user sync status instead of the global catalog

The header status bar read the global /api/sync/status (videos_total +
channels_backfilling), so every user saw the whole catalog's numbers —
confusing and a small cross-user info leak (e.g. "3 syncing" for a user with 2
channels). It now uses /api/sync/my-status: the user's own available video count
and how many of their own channels are still being fetched
(channels_recent_pending). The pause control stays admin-only via an isAdmin prop.

frontend/src/components/Header.tsx

@@ -36,7 +36,7 @@ export default function Header({
         Sift<span className="text-accent">lode</span>
       </button>
 
-      <SyncStatus />
+      <SyncStatus isAdmin={me.role === "admin"} />
 
       {page === "feed" ? (
         <div className="flex-1 max-w-xl mx-auto relative">