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/App.tsx

@@ -24,6 +24,7 @@ import Header from "./components/Header";
 import Sidebar from "./components/Sidebar";
 import Feed from "./components/Feed";
 import Channels, { type ChannelStatusFilter } from "./components/Channels";
+import Playlists from "./components/Playlists";
 import Stats from "./components/Stats";
 import SettingsPanel from "./components/SettingsPanel";
 import OnboardingWizard from "./components/OnboardingWizard";
@@ -218,6 +219,8 @@ export default function App() {
             />
           ) : page === "stats" && meQuery.data!.role === "admin" ? (
             <Stats />
+          ) : page === "playlists" ? (
+            <Playlists />
           ) : (
             <Feed
               filters={filters}