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/images/close-button.svg

@@ -0,0 +1,14 @@
+<svg width="32" height="32" viewBox="0 0 32 32" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <g clip-path="url(#clip0_373_731)">
+        <path d="M25 7L7 25" stroke="#F5F5F5" stroke-width="2" stroke-linecap="round"
+            stroke-linejoin="round" />
+        <path d="M25 25L7 7" stroke="#F5F5F5" stroke-width="2" stroke-linecap="round"
+            stroke-linejoin="round" />
+    </g>
+    <defs>
+        <clipPath id="clip0_373_731">
+            <rect width="32" height="32" fill="white" />
+        </clipPath>
+    </defs>
+</svg>
+