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/lib/storage.ts

@@ -6,6 +6,9 @@ import { useCallback, useState } from "react";
 export const LS = {
   theme: "siftlode.theme",
   sidebarLayout: "siftlode.sidebarLayout",
+  plexLayout: "siftlode.plexLayout",
+  playlistsLayout: "siftlode.playlistsLayout",
+  playlistsCollapsed: "siftlode.playlistsCollapsed",
   hints: "siftlode.hints",
   lang: "siftlode.lang",
   filters: "siftlode.filters",