feat(web/admin): sticky tabs at top of admin scroll container

The admin tab strip (Overview / Integrations / Requests / Quarantine)
scrolled out of view as the operator paged through long sections like
Library Scan, Cover Art, etc. Pin it to the top of the <main>
scroll container with sticky positioning, an opaque background to
hide content scrolling behind it, and z-10 to layer above section
content.

web/src/lib/components/AdminTabs.svelte

@@ -19,7 +19,7 @@
   }
 </script>
 
-<nav aria-label="Admin sections" class="border-b border-border">
+<nav aria-label="Admin sections" class="sticky top-0 z-10 border-b border-border bg-background">
   <ul class="flex gap-2">
     {#each items as item (item.href)}
       <li>