feat(layout): a pinned filter rail no longer pushes content when there's room for it

On a page that centres its content at a max width, if the window is wide enough for the rail to fit
in the content's LEFT margin without overlapping it, pinning the rail now leaves it OVERLAY — the
content doesn't shift. It reverts to the old in-flow "push" only when there isn't room: a full-width
page (feed) or a window too narrow for the rail to clear the centred content.

PageShell computes --rail-pinned-w from the page's maxWidth vs the viewport (innerWidth >= maxWidth +
~576px → overlay 2.25rem, else in-flow 16.75rem) on a resize listener; the SidePanel slot reads it.

Verified at 1920px: pinning the playlists rail leaves the table at left 487 (moved 0); the rail sits
256px wide ending at 312 — 175px clear of the content. Feed (full width) still pins in-flow.

frontend/src/components/SidePanel.tsx

@@ -44,14 +44,16 @@ export default function SidePanel({
 
   return (
     // Layout slot: reserves only the tab's gutter when unpinned, so the expanded card overlays
-    // content instead of pushing it. Pinned reserves the full width (in-flow), as before.
+    // content instead of pushing it. Pinned reserves --rail-pinned-w: 16.75rem (in-flow, pushes) on a
+    // full-width page or a narrow window, but 2.25rem (overlay, doesn't push) when the centred content
+    // leaves room for the rail to sit in its left margin — see PageShell. Default 16.75rem = old w-67.
     <div
       // Focus is tracked container-wide (keyboard: tab anywhere inside holds it open). HOVER is NOT
       // on this full-height slot — that made the whole column next to the nav rail a live target, so
       // merely approaching the rail popped the panel open. Hover now lives only on the visible tab
       // handle + the expanded card below, so only pointing AT the tab opens it.
       {...peek.focusHandlers}
-      className={`hidden md:block relative shrink-0 transition-[width] ${pinned ? "w-67" : "w-9"}`}
+      className={`hidden md:block relative shrink-0 transition-[width] ${pinned ? "w-[var(--rail-pinned-w,16.75rem)]" : "w-9"}`}
     >
       {/* The tab: a small handle flush against the nav rail, flat on the docked edge. It stays
           mounted while the card is open — swapping it out from under the pointer would drop the very