Shorten thread route from /thread to /t

All internal links (tiles, prev/next nav, comments, notifications,
breadcrumb, dropzone route check) point to /t/<id>. A global route
middleware permanently redirects old /thread/<id> URLs including
query string and comment anchor.

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

app/components/SiteBreadcrumb.vue

@@ -20,7 +20,7 @@ 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') })
+  if (p.startsWith('/t/')) 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') })