feat(uat-batch-17): layout polish — DocumentsHub flush-left, breadcrumb wrap fix, viewport-centered topbar search

- DocumentsHub root container gains `sm:-mx-6 sm:-mt-3 sm:-mb-6` to
  escape the AppShell main padding (`px-6 pt-3 pb-6`). The folder
  column now sits flush against the global app sidebar, reading as an
  extension of navigation rather than a card-inside-a-page. Mobile
  layout retains the AppShell padding.
- Breadcrumbs: each crumb + its trailing separator now share a single
  `<BreadcrumbItem>` instead of being separate `<li>`s. Flex-wrap can
  no longer strand an orphan separator at end-of-line above a wrapped
  child crumb. Drops the standalone `<BreadcrumbSeparator>` usage from
  the consumer; the primitive is still exported for backcompat.
- Topbar search visually centered against the full viewport via a
  `translate-x:calc(-var(--width-sidebar)/2)` shift. Grid middle slot
  bumped from `minmax(360px, 640px)` → `minmax(420px, 800px)` and the
  search wrapper from `max-w-md` → `max-w-2xl` so reps actually have
  room to read long results.

tsc clean. 1419/1419 vitest pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/documents/documents-hub.tsx

@@ -243,7 +243,12 @@ export function DocumentsHub({ portSlug }: DocumentsHubProps) {
   );
 
   return (
-    <div className="h-full">
+    // Escape the AppShell's desktop main padding (px-6 pt-3 pb-6) so the
+    // folder column sits flush against the global app sidebar — reads
+    // as an extension of navigation rather than a card-inside-a-page.
+    // Inner content keeps its own padding so the right pane doesn't
+    // run flush with the viewport edge.
+    <div className="h-full sm:-mx-6 sm:-mt-3 sm:-mb-6">
       {/* Mobile: stacked, with a Sheet drawer for folders. */}
       <div className="flex flex-col h-full sm:hidden">
         <FolderTreeSidebar