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/i18n/locales/en/header.json

@@ -14,6 +14,7 @@
     "admin": "Admin",
     "feed": "Feed",
     "channels": "Channels",
+    "playlists": "Playlists",
     "stats": "Stats",
     "settings": "Settings",
     "about": "About",

frontend/src/i18n/locales/en/playlists.json

@@ -0,0 +1,15 @@
+{
+  "title": "Playlists",
+  "noneYet": "No playlists yet",
+  "newPlaylist": "New playlist…",
+  "itemCount_one": "{{count}} video",
+  "itemCount_other": "{{count}} videos",
+  "pickOne": "Pick a playlist on the left.",
+  "loading": "Loading…",
+  "empty": "This playlist is empty — add videos from the feed.",
+  "playAll": "Play all",
+  "delete": "Delete",
+  "rename": "Rename",
+  "confirmDelete": "Delete the playlist “{{name}}”? This can't be undone.",
+  "addToPlaylist": "Add to playlist"
+}