refactor(docs): drop dead grid placement, move footer stacking to the component

Review follow-ups. The divider's `grid-row`/`grid-column` stopped doing anything
the moment it became `position: fixed` — a fixed box is out of grid layout
entirely — so they and the comment explaining the grid span were describing
positioning that no longer happens. Verified inert: the divider still computes
to left 300px / width 1px / z-index 21 without them.

The footer's stacking context also belongs on the footer, not in a global rule
matching every desktop `footer` element, so it moves to the component as
`relative z-[22]` with the reason in its TSDoc.

Author: waleedlatif1

apps/docs/app/global.css

@@ -380,17 +380,14 @@ aside#nd-sidebar [data-radix-scroll-area-viewport] {
     width: var(--fd-sidebar-width);
   }
 
-  /* Sidebar divider line — pinned for the same reason, and to stay glued to the
-     sidebar's right edge. #nd-docs-layout is a CSS grid (see fumadocs' Container
-     slot); the pseudo-element spans the full grid so it reads as decorative
-     overlay rather than being auto-placed into a real content cell and skewing
-     that cell's sizing. */
+  /* Sidebar divider line — pinned for the same reason, and so it stays glued to
+     the sidebar's right edge. Being fixed takes it out of #nd-docs-layout's grid
+     entirely, so it needs no grid placement and cannot skew a content cell; its
+     position comes from `left`/`top` alone. */
   #nd-docs-layout::before {
     content: "";
     display: block;
     position: fixed;
-    grid-row: 1 / -1;
-    grid-column: 1 / -1;
     top: 92px; /* below navbar */
     height: calc(100dvh - 92px);
     left: calc(var(--sidebar-offset) + var(--fd-sidebar-width));
@@ -400,14 +397,6 @@ aside#nd-sidebar [data-radix-scroll-area-viewport] {
     z-index: 21;
   }
 
-  /* The footer is opaque and now has to out-stack the pinned sidebar and its
-     divider (z-index 20 and 21), so it slides over them at the end of the page
-     rather than being drawn through. */
-  footer {
-    position: relative;
-    z-index: 22;
-  }
-
   /* Hide fumadocs nav on desktop - we use custom navbar there */
   #nd-docs-layout > header {
     display: none !important;

apps/docs/components/footer/footer.tsx

@@ -130,9 +130,16 @@ function FooterColumn({ title, items }: { title: string; items: FooterItem[] })
   )
 }
 
+/**
+ * Site footer.
+ *
+ * `relative z-[22]` stacks it above the docs sidebar (z-20) and that sidebar's
+ * divider (z-21), both of which are pinned to the viewport, so the footer slides
+ * over them at the end of the page instead of being drawn through.
+ */
 export function Footer() {
   return (
-    <footer className='mt-[120px] w-full border-[var(--border)] border-t bg-[var(--bg)] max-sm:mt-16 max-lg:mt-[88px]'>
+    <footer className='relative z-[22] mt-[120px] w-full border-[var(--border)] border-t bg-[var(--bg)] max-sm:mt-16 max-lg:mt-[88px]'>
       <div className='mx-auto w-full max-w-[1460px] px-20 pt-16 pb-16 max-sm:px-5 max-lg:px-8 max-lg:pt-12 max-lg:pb-12'>
         <nav
           aria-label='Footer navigation'