fix(ux): dock filter tab flush to the nav + reliable full-viewport plex backdrop

Follow-ups on the 7-item batch:
- The slim nav slot was still w-20 while its card is w-14, leaving a ~24px dead
  gap the filter tab/bar couldn't dock against. Slot is now w-14 (exactly the
  card width), so the filter's flat-left tab sits flush on the rail's right edge
  (measured gap 0), both collapsed and expanded.
- The Plex art backdrop was invisible: a position:fixed layer at z-index -1 is
  painted BEHIND the body's opaque background here, so nothing showed. Paint the
  art on the app root (#root) instead — it's exactly the viewport and a real
  ancestor of the transparent nav/header/content, so the art fills the whole page
  edge-to-edge and the glass chrome floats over it (verified in-browser).

frontend/src/components/NavSidebar.tsx

@@ -201,7 +201,8 @@ export default function NavSidebar({
     // Layout slot: reserves only the slim footprint when unpinned, so the expanded rail (below)
     // overlays content instead of pushing it. Pinned reserves the full width (in-flow).
     // Slim icon rail, flush against the left viewport edge (only top/bottom margin), never grows.
-    <div className="relative shrink-0 w-20">
+    // The slot is exactly the card's width so the filter tab/bar docks flush against its right edge.
+    <div className="relative shrink-0 w-14">
       <nav
         className="glass absolute top-0 bottom-0 left-0 z-rail rounded-2xl flex flex-col py-3 my-3 w-14 px-2"
         aria-label={t("nav.primary")}