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/chevron.svg

@@ -0,0 +1,22 @@
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+    viewBox="0 0 32 32"
+    fill="none"
+    xmlns="http://www.w3.org/2000/svg"
+>
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+        <path
+            d="M26 12L16 22L6 12"
+            stroke="#625B6B"
+            stroke-width="1.5"
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+            stroke-linejoin="round"
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+    <defs>
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