feat(ui): unified floating side panels for filters and playlists

Replace the full-height filter rails with one shared floating glass SidePanel
used by Feed, Plex and Playlists. Each collapses to a slim tab beside the nav;
its sections are reorderable "islands" (drag / hide / per-group collapse) with a
per-panel saved layout; the body hides its scrollbar with a soft top/bottom edge
fade. The nav rail becomes a matching floating rounded card.

- New shared SidePanel / PanelGroup / PanelGroups + useScrollFade hook.
- Generalise the feed-only sidebarLayout into a per-panel panelLayout (feed keeps
  its storage key; plex/playlists get their own; persisted per account).
- Lift the Playlists rail to App level (selected playlist is now App state) so it
  floats and collapses like the filter rails; remove the old CollapsedFilterRail.
- The floating top header's fixed offset accounts for the panels' margins.

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

@@ -1,5 +1,7 @@
 {
   "title": "Playlists",
+  "manageGroup": "Manage",
+  "listGroup": "Your playlists",
   "watchLater": "Watch later",
   "syncYoutube": "Sync from YouTube",
   "syncedToast": "Synced {{count}} playlists from YouTube",

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

@@ -1,5 +1,7 @@
 {
   "title": "Lejátszási listák",
+  "manageGroup": "Kezelés",
+  "listGroup": "Listáid",
   "watchLater": "Megnézem később",
   "syncYoutube": "Szinkron YouTube-ról",
   "syncedToast": "{{count}} lista szinkronizálva a YouTube-ról",