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/mssql/telemetry_mssql.sql

@@ -0,0 +1,27 @@
+--
+-- MSSQL database schema for speedtest telemetry
+--
+
+CREATE TABLE [dbo].[speedtest_users](
+    [id]        [bigint] IDENTITY(120,1) NOT NULL,
+    [timestamp] [datetime] NOT NULL,
+    [ip]        [nvarchar](max) NOT NULL,
+    [ispinfo]   [nvarchar](max) NULL,
+    [extra]     [nvarchar](max) NULL,
+    [ua]        [nvarchar](max) NOT NULL,
+    [lang]      [nvarchar](max) NOT NULL,
+    [dl]        [nvarchar](max) NULL,
+    [ul]        [nvarchar](max) NULL,
+    [ping]      [nvarchar](max) NULL,
+    [jitter]    [nvarchar](max) NULL,
+    [log]       [nvarchar](max) NULL,
+    [uuid]      [nvarchar](max) NULL,
+ CONSTRAINT [PK_speedtest_users] PRIMARY KEY CLUSTERED
+(
+    [id] ASC
+) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
+) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]
+GO
+
+ALTER TABLE [dbo].[speedtest_users] ADD  CONSTRAINT [DF_speedtest_users_timestamp]  DEFAULT (getdate()) FOR [timestamp]
+GO