fix: LND UI use protocol-aware fetch, default backend URL

- fetchConnectInfo: use window.location.protocol instead of hardcoded http://
- getBackendUrl: default to current origin when no ?backend= param
- Fixes mixed content errors on HTTPS Tailscale servers
- Also fixed: nginx needed reload on Tailscale servers, Arch 2 missing
  /lnd-connect-info nginx location

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

docker/lnd-ui/index.html

@@ -434,7 +434,7 @@
 
         function getBackendUrl() {
             const params = new URLSearchParams(window.location.search);
-            return params.get('backend') || '';
+            return params.get('backend') || (window.location.protocol + '//' + window.location.hostname);
         }
 
         function setSettingsTab(tabId) {
@@ -629,7 +629,7 @@
 
         async function fetchConnectInfo() {
             try {
-                const resp = await fetch('http://' + window.location.hostname + '/lnd-connect-info');
+                const resp = await fetch(window.location.protocol + '//' + window.location.hostname + '/lnd-connect-info');
                 if (!resp.ok) throw new Error('HTTP ' + resp.status);
                 const data = await resp.json();
                 if (data.cert_base64url) {