feat(plex): total playtime in the playlist header (E9 S3)

Parity with the YouTube playlist header (E9 12D): the Plex playlist detail
now carries a meta line under the title — "N items · Xh Ym", summed from the
items' duration_seconds. No backend change: leaf_card already serializes
duration_seconds for both movies and episodes, and no new i18n key either —
the count reuses plex.browse.items, the same key the collection header uses.

The title/rename input moved into a min-w-0 flex-1 column so the meta line
sits under both; the line is hidden on an empty playlist.

frontend/src/components/PlexPlaylistView.tsx

@@ -32,6 +32,7 @@ import {
   X,
 } from "lucide-react";
 import { api, type PlexCard } from "../lib/api";
+import { formatRuntime } from "../lib/format";
 import { getAccountRaw, LS, setAccountRaw } from "../lib/storage";
 import { useConfirm } from "./ConfirmProvider";
 import { LoadingState, StateMessage } from "./QueryState";
@@ -387,6 +388,12 @@ export default function PlexPlaylistView({
   }, [q.data]);
   const blocks = useMemo(() => buildBlocks(items), [items]);
   const order = useMemo(() => items.map((i) => i.id), [items]);
+  // Total playtime of the whole playlist, mirroring the YouTube playlist header (E9 12D): items with
+  // an unknown duration count as 0, and the runtime is dropped from the meta line when nothing sums.
+  const totalSeconds = useMemo(
+    () => items.reduce((acc, it) => acc + (it.duration_seconds ?? 0), 0),
+    [items],
+  );
 
   const invalidate = () => {
     qc.invalidateQueries({ queryKey: qk.plexPlaylist(playlistId) });
@@ -517,18 +524,26 @@ export default function PlexPlaylistView({
 
       <div className="glass rounded-2xl p-4 sm:p-6">
         <div className="mb-4 flex flex-wrap items-center gap-3">
-          {renaming ? (
-            <input
-              autoFocus
-              value={name}
-              onChange={(e) => setName(e.target.value)}
-              onKeyDown={(e) => e.key === "Enter" && rename()}
-              onBlur={rename}
-              className="glass-card min-w-0 flex-1 rounded-lg px-3 py-1.5 text-lg font-bold focus:outline-hidden focus:ring-2 focus:ring-accent/40"
-            />
-          ) : (
-            <h1 className="min-w-0 flex-1 truncate text-2xl font-bold">{q.data?.title ?? " "}</h1>
-          )}
+          <div className="min-w-0 flex-1">
+            {renaming ? (
+              <input
+                autoFocus
+                value={name}
+                onChange={(e) => setName(e.target.value)}
+                onKeyDown={(e) => e.key === "Enter" && rename()}
+                onBlur={rename}
+                className="glass-card w-full rounded-lg px-3 py-1.5 text-lg font-bold focus:outline-hidden focus:ring-2 focus:ring-accent/40"
+              />
+            ) : (
+              <h1 className="truncate text-2xl font-bold">{q.data?.title ?? " "}</h1>
+            )}
+            {items.length > 0 && (
+              <div className="mt-0.5 text-sm text-muted">
+                {t("plex.browse.items", { count: items.length })}
+                {totalSeconds > 0 && ` · ${formatRuntime(totalSeconds)}`}
+              </div>
+            )}
+          </div>
           {/* Layout preference: Accordion or Tree — the shared list-view switcher. */}
           <ViewSwitcher
             value={layout}