Header: replace My Account nav link with an account icon in the top right (sign-in link / account menu), collapse header below 900px; v2.23.0

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.22.1",
+  "version": "2.23.0",
   "type": "module",
   "scripts": {
     "dev": "vite",