feat(playlists): frontend foundation — page, add-to-playlist, nav

Add the Playlists page (left rail of playlists + detail with drag&drop reorder,
inline rename, one-step delete, remove item, Play all / row-click playback via the
existing PlayerModal) and an AddToPlaylist popover (portaled, multi-toggle +
inline new-playlist) wired into the VideoCard hover overlay and the PlayerModal.
New api client methods + Playlist types, a 'playlists' page route + account-menu
entry, and trilingual strings. Local only — YouTube sync comes in later phases.

frontend/src/components/VideoCard.tsx

@@ -11,6 +11,7 @@ import {
   RotateCcw,
 } from "lucide-react";
 import Avatar from "./Avatar";
+import AddToPlaylist from "./AddToPlaylist";
 import clsx from "clsx";
 import type { Video } from "../lib/api";
 import { formatDuration, formatViews, relativeTime } from "../lib/format";
@@ -56,6 +57,7 @@ function Actions({
       >
         <Bookmark className="w-4 h-4" />
       </button>
+      <AddToPlaylist videoId={video.id} />
       <button
         onClick={act("hidden")}
         title={video.status === "hidden" ? t("card.unhide") : t("card.hide")}