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

database/sqlite/telemetry_sqlite.sql

@@ -0,0 +1,21 @@
+--
+-- SQLite database schema for speedtest telemetry
+-- Auto-created by the sqlite backend if it doesn't exist.
+-- This file is provided for reference / manual setup.
+--
+
+CREATE TABLE IF NOT EXISTS `speedtest_users` (
+    `id`        INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+    `timestamp` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    `ip`        TEXT NOT NULL,
+    `ispinfo`   TEXT,
+    `extra`     TEXT,
+    `ua`        TEXT NOT NULL,
+    `lang`      TEXT NOT NULL,
+    `dl`        TEXT,
+    `ul`        TEXT,
+    `ping`      TEXT,
+    `jitter`    TEXT,
+    `log`       TEXT,
+    `uuid`      TEXT
+);