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/layouts/default.vue

@@ -1,7 +1,9 @@
 <script setup lang="ts">
 import CommentsSidebar from '@/components/CommentsSidebar.vue'
 import CommandPalette from '@/components/CommandPalette.vue'
+import GlobalDropZone from '@/components/GlobalDropZone.vue'
 import MotdBanner from '@/components/MotdBanner.vue'
+import SiteBreadcrumb from '@/components/SiteBreadcrumb.vue'
 import SiteHeader from '@/components/SiteHeader.vue'
 import {
   SidebarInset,
@@ -20,6 +22,7 @@ import {
     <SidebarInset>
       <SiteHeader />
       <MotdBanner />
+      <SiteBreadcrumb />
       <div class="flex flex-1 flex-col">
         <div class="@container/main flex flex-1 flex-col gap-2">
           <div class="flex flex-1 flex-col gap-4 py-4 md:gap-6 md:py-6">
@@ -30,5 +33,6 @@ import {
     </SidebarInset>
     <CommentsSidebar />
     <CommandPalette />
+    <GlobalDropZone />
   </SidebarProvider>
 </template>