feat: add canonical URL and og:url

Docus doesn't generate canonical links or og:url natively. A plugin
now injects rel=canonical and og:url per route using nuxt-site-config's
withSiteUrl, which reads site.url from nuxt.config.ts.

plugins/canonical.ts

@@ -0,0 +1,13 @@
+export default defineNuxtPlugin(() => {
+  const route = useRoute()
+  const canonicalUrl = withSiteUrl(computed(() => route.path), { canonical: true })
+
+  useHead({
+    link: [
+      { rel: 'canonical', href: canonicalUrl }
+    ],
+    meta: [
+      { property: 'og:url', content: canonicalUrl }
+    ]
+  })
+})