Per-hostname data directory <hostname>/{content.kantadb, analytics.json, files} from CLI arg; drop learned site_url.

The first positional CLI argument (default localhost) names the site's
public hostname and its data directory under the cwd, replacing the
CWD-relative pagerite.* files. The public origin (https://<hostname>)
is now authoritative configuration instead of a value learned from
admin browsers: POST /_api/site-url, Data.site_url and the pagerite.js
reporter are removed, and page rendering, sitemap, robots.txt and
analytics own_origin all use SITE_URL consistently (localhost falls
back to the request's base URL).

frontend/src/analytics/format.js

@@ -152,6 +152,15 @@ function externalSlug(origin) {
   }
 }
 
+/** Origin (scheme://host) of an external https URL, for favicon lookup. */
+function externalOrigin(url) {
+  try {
+    return new URL(url).origin
+  } catch {
+    return ''
+  }
+}
+
 /** Format one trail step: an internal page or an external https origin. */
 function stepOf(path, titles) {
   if (path?.startsWith('/')) {
@@ -163,6 +172,7 @@ function stepOf(path, titles) {
       slug: externalSlug(path),
       title: 'External site',
       external: true,
+      origin: externalOrigin(path),
     }
   }
   return null