Sync PHP backend feature parity: IP detection, database backends, API endpoints, and frontend

- IP detection: Cloudflare IPv6, ULA IPv6, proxy header chain, offline GeoIP DB
- Database: add SQLite (pure Go, no CGo) and MSSQL backends
- API: add JSON result sharing endpoint and ID obfuscation
- Frontend: add modern CSS design, design switcher, favicon
- Compatibility: ?cors parameter support, human-friendly distance rounding
- Update Go to 1.21, add modernc.org/sqlite and maxminddb deps

web/assets/design-switch.js

@@ -0,0 +1,45 @@
+/**
+ * Feature switch for enabling the new LibreSpeed design
+ *
+ * This script checks for:
+ * 1. URL parameter: ?design=new or ?design=old
+ * 2. Default behavior: Shows the classic design
+ *
+ * Note: This script is only loaded on the root index.html
+ */
+(function () {
+    'use strict';
+
+    // Don't run this script if we're already on a specific design page
+    const currentPath = window.location.pathname;
+    if (currentPath.includes('index-classic.html') || currentPath.includes('index-modern.html')) {
+        return;
+    }
+
+    // Check URL parameters first
+    const urlParams = new URLSearchParams(window.location.search);
+    const designParam = urlParams.get('design');
+
+    if (designParam === 'new') {
+        redirectToNewDesign();
+        return;
+    }
+
+    if (designParam === 'old' || designParam === 'classic') {
+        redirectToOldDesign();
+        return;
+    }
+
+    // Default to classic design
+    redirectToOldDesign();
+
+    function redirectToNewDesign() {
+        const currentParams = window.location.search;
+        window.location.href = 'index-modern.html' + currentParams;
+    }
+
+    function redirectToOldDesign() {
+        const currentParams = window.location.search;
+        window.location.href = 'index-classic.html' + currentParams;
+    }
+})();