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/middleware/thread-redirect.global.ts

@@ -0,0 +1,6 @@
+// Alter Thread-Pfad — /thread/<id> leitet dauerhaft auf /t/<id> um
+export default defineNuxtRouteMiddleware((to) => {
+  if (to.path.startsWith('/thread/')) {
+    return navigateTo({ path: to.path.replace(/^\/thread\//, '/t/'), query: to.query, hash: to.hash }, { redirectCode: 301 })
+  }
+})