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/index.css

@@ -113,6 +113,16 @@ body {
   background-attachment: fixed;
 }
 
+/* Hide the native scrollbar while keeping the element scrollable (wheel/trackpad/keyboard).
+   Used on the nav rail so a high browser-zoom overflow scrolls without showing a scrollbar. */
+.no-scrollbar {
+  scrollbar-width: none; /* Firefox */
+  -ms-overflow-style: none; /* legacy Edge/IE */
+}
+.no-scrollbar::-webkit-scrollbar {
+  display: none; /* Chromium/WebKit */
+}
+
 /* ===== Liquid-glass surface system (theme-aware, GPU-light) ===== */
 .glass {
   /* Frosted overlay surface: translucent enough that the blurred backdrop shows