Layout additions: breadcrumbs, global drop zone, sticky MOTD

- Route-derived SiteBreadcrumb below the MOTD on all non-index
  pages; separator icon switched to bxs:chevron-right-square
- GlobalDropZone moved to the layout so it works on every page;
  on thread routes it only applies to the upper half of the
  viewport (comment attachments keep their own zone)
- MOTD banner is now sticky below the site header

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

app/components/MotdBanner.vue

@@ -7,7 +7,7 @@ if (data.value.message) motd.value = data.value.message
 </script>
 
 <template>
-  <div v-if="motd" class="border-b px-4 py-2 lg:px-6">
+  <div v-if="motd" class="sticky top-(--header-height) z-10 border-b bg-background px-4 py-2 lg:px-6">
     <p class="flex items-center gap-2 text-sm">
       <Icon name="ph:warning-octagon-fill" class="size-4 shrink-0 text-muted-foreground" />
       <span class="min-w-0 whitespace-pre-wrap">{{ motd }}</span>