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/SiteBreadcrumb.vue

@@ -0,0 +1,52 @@
+<script setup lang="ts">
+import {
+  Breadcrumb,
+  BreadcrumbItem,
+  BreadcrumbLink,
+  BreadcrumbList,
+  BreadcrumbPage,
+  BreadcrumbSeparator,
+} from '@/components/ui/breadcrumb'
+
+const { t } = useI18n()
+const route = useRoute()
+
+interface Crumb {
+  label: string
+  to?: string
+}
+
+const crumbs = computed<Crumb[]>(() => {
+  const list: Crumb[] = [{ label: t('palette.home'), to: '/' }]
+  const p = route.path
+  if (p === '/') return []
+  if (p.startsWith('/thread/')) list.push({ label: t('nav.thread') })
+  else if (p === '/users') list.push({ label: t('nav.users') })
+  else if (p.startsWith('/user/')) {
+    list.push({ label: t('nav.users'), to: '/users' }, { label: t('nav.profile') })
+  }
+  else if (p === '/settings') list.push({ label: t('nav.settings') })
+  else if (p === '/admin') list.push({ label: t('nav.admin') })
+  else if (p === '/login') list.push({ label: t('auth.login') })
+  else if (p === '/register') list.push({ label: t('auth.register') })
+  return list
+})
+</script>
+
+<template>
+  <Breadcrumb v-if="crumbs.length > 1" class="px-4 py-2 lg:px-6">
+    <BreadcrumbList>
+      <template v-for="(crumb, i) in crumbs" :key="i">
+        <BreadcrumbItem>
+          <BreadcrumbLink v-if="crumb.to" as-child>
+            <NuxtLink :to="crumb.to">{{ crumb.label }}</NuxtLink>
+          </BreadcrumbLink>
+          <BreadcrumbPage v-else>
+            {{ crumb.label }}
+          </BreadcrumbPage>
+        </BreadcrumbItem>
+        <BreadcrumbSeparator v-if="i < crumbs.length - 1" />
+      </template>
+    </BreadcrumbList>
+  </Breadcrumb>
+</template>