feat(playlists): total playtime in header + sticky-header fade fix (E9 S1)

Surface the playlist's total playtime beside the item count in the detail
header (12D) — sums every item's duration (formatRuntime "h m", matching the
approved mockup), hidden when nothing sums. Data is already client-side; no
backend change.

Pass fadeTop:false for the playlists page (12B) so the shell's top edge-fade
no longer sits over the already-sticky column header (tableHeader.tsx:257),
matching the channels/audit pages.

frontend/src/components/Playlists.tsx

@@ -39,6 +39,7 @@ import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { playlistName } from "../lib/playlistName";
 import { useUndoable } from "../lib/useUndoable";
 import { cycleSort, type SortState } from "../lib/columnSort";
+import { formatRuntime } from "../lib/format";
 import { applyView, sameOrder } from "../lib/playlistView";
 const PlayerModal = lazy(() => import("./PlayerModal"));
 import UndoToolbar from "./UndoToolbar";
@@ -259,6 +260,13 @@ export default function Playlists() {
     return (v: Video) => m.get(v.id) ?? 0;
   }, [items]);
 
+  // Total playtime of the whole playlist (sums every item, matching the itemCount beside it — not
+  // the filtered `shown` set). Items with an unknown duration count as 0; hidden when nothing sums.
+  const totalSeconds = useMemo(
+    () => items.reduce((acc, v) => acc + (v.duration_seconds ?? 0), 0),
+    [items],
+  );
+
   const columns = useMemo(
     () => playlistColumns({ t, indexOf: storedIndex, onPlay, onRemove, canEdit }),
     [t, storedIndex, onPlay, onRemove, canEdit],
@@ -648,6 +656,7 @@ export default function Playlists() {
               )}
               <div className="text-xs text-muted mt-0.5">
                 {t("playlists.itemCount", { count: items.length })}
+                {totalSeconds > 0 && ` · ${formatRuntime(totalSeconds)}`}
                 {filterActive && ` · ${t("playlists.filteredCount", { count: shown.length })}`}
               </div>
               <div className="flex gap-1.5 mt-3 flex-wrap items-center">

frontend/src/components/moduleRegistry.tsx

@@ -94,7 +94,7 @@ export type PageDef = {
 export const PAGE_CONTENT: Record<Page, PageDef> = {
   feed: { Component: FeedPage },
   channels: { Component: Channels, fadeTop: false, maxWidth: "96rem" },
-  playlists: { Component: Playlists, maxWidth: "64rem" },
+  playlists: { Component: Playlists, fadeTop: false, maxWidth: "64rem" },
   plex: { Component: PlexBrowse, maxWidth: "1600px" },
   notifications: { Component: NotificationsPanel, maxWidth: "48rem" },
   messages: { Component: MessagesPage },