feat(threads): add SEO metadata, sitemap, and structured data

Include /threads in sitemap and set canonical paths
Generate DiscussionForumPosting structured data for thread details
Add dynamic SEO updates for thread navigation and server-side rendering

frontend/src/views/ThreadsView.vue

@@ -22,6 +22,7 @@ import {
   iconThreads,
   iconUndo
 } from '../icons';
+import { applySeo, resolvedSeoHead, resolveSeo, threadSeoConfig } from '../seo';
 import {
   api,
   threadWebSocketUrl,
@@ -158,6 +159,9 @@ const currentThreadList = computed(() => {
   });
 });
 const detailModalOpen = computed(() => activeThread.value !== null);
+const threadSeo = computed(() => (activeThread.value ? resolveSeo(threadSeoConfig(activeThread.value, t)) : null));
+
+useHead(() => (threadSeo.value ? resolvedSeoHead(threadSeo.value) : {}));
 
 const messageGroups = computed<MessageGroup[]>(() => {
   const groups: MessageGroup[] = [];
@@ -826,6 +830,12 @@ watch(activeThreadId, async () => {
   await loadMessages(true);
 });
 
+watch(activeThread, (thread) => {
+  if (thread) {
+    applySeo(threadSeoConfig(thread, t));
+  }
+});
+
 onMounted(() => {
   void loadAll();
 });