feat(server): fall back Sentinel URL to request host

Use the current request scheme/host when no FQDN or public IP is set,
skipping loopback hosts. Polish boarding/server create cards, optional
listbox tooltips, and cover the layout and Sentinel URL behavior in tests.

tests/Unit/StartSentinelEndpointFallbackTest.php

@@ -10,7 +10,8 @@ test('sentinel startup regenerates an empty endpoint from instance settings', fu
         ->toContain('$this->sentinelCustomUrl = $this->server->settings->sentinel_custom_url;')
         ->and(file_get_contents(dirname(__DIR__, 2).'/app/Models/ServerSetting.php'))
         ->toContain('generateSentinelUrl(ignoreEvent: true)')
-        ->toContain('Set an instance FQDN or public IP before enabling Sentinel.')
+        ->toContain('sentinelUrlFromCurrentRequest()')
+        ->toContain('Set an instance FQDN, public IP, or reachable Coolify URL before enabling Sentinel.')
         ->and($component)
         ->toContain('$this->sentinelCustomUrl = $this->server->settings->sentinel_custom_url;');
 });