fix(playlists): correctly clear/reselect after deleting a playlist

The post-delete auto-select effect re-picked the just-deleted id from the still-stale
playlists cache, so its detail lingered. Select the next remaining playlist directly on
delete (or null), drop the deleted detail from the cache, and make the effect re-validate
the selection against the current list (clear when empty, reselect when the current id is
gone). Also fix the header title showing "Channel manager" on the Playlists page.

frontend/src/components/Header.tsx

@@ -90,7 +90,11 @@ export default function Header({
         </div>
       ) : (
         <div className="flex-1 text-center text-sm font-semibold">
-          {page === "stats" ? t("header.usageStats") : t("header.channelManager")}
+          {page === "stats"
+            ? t("header.usageStats")
+            : page === "playlists"
+              ? t("header.account.playlists")
+              : t("header.channelManager")}
         </div>
       )}