feat: single-domain path-based routing (no subdomains required)

Move SPA assets from /assets/ to /_app/ (Vite assetsDir config) so
Traefik can route /assets/* to Logto without conflict. All services
on one hostname with path-based routing:

- /oidc/*, /interaction/*, /assets/* → Logto
- /server/* → server-ui (prefix stripped)
- /api/* → cameleer-saas
- /* (catch-all) → cameleer-saas SPA

Customer needs only 1 DNS record. Server gets OIDC_JWK_SET_URI for
Docker-internal JWK fetch (standard Spring split config).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/components/Layout.tsx

@@ -162,7 +162,7 @@ export function Layout() {
         <Sidebar.FooterLink
           icon={<ObsIcon />}
           label="View Dashboard"
-          onClick={() => window.open(`${window.location.protocol}//server.${window.location.hostname}`, '_blank', 'noopener')}
+          onClick={() => window.open('/server/', '_blank', 'noopener')}
         />
 
         {/* User info + logout */}