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/components/Header.tsx

@@ -103,7 +103,9 @@ export default function Header({
   }
 
   return (
-    <div className="pointer-events-none fixed top-3 left-4 right-[10%] z-30 flex items-center gap-3 md:left-[480px]">
+    // Fixed left = the left zone at its widest: nav card (12 margin + 208 + 12 margin = 232) +
+    // filter sidebar (256) + 16 gap = 504. Constant, so it never shifts on collapse/module change.
+    <div className="pointer-events-none fixed top-3 left-4 right-[10%] z-30 flex items-center gap-3 md:left-[504px]">
       <ModuleName
         label={t(moduleLabelKey[page])}
         labels={moduleLabels}