Add SK Broadband SNI block pattern (TLS disconnect before handshake)

SK uses different error than KT/LG: "socket disconnected before secure
TLS connection was established" instead of ECONNRESET. Now all 3 Korean
ISPs (KT, LG U+, SK) are correctly detected.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/services/network-diagnostic.ts

@@ -322,7 +322,7 @@ async function pollGlobalpingResult(id: string): Promise<GlobalpingRawResult[]>
 }
 
 // SNI 차단 시 나타나는 에러 패턴 (한국 ISP DPI)
-const SNI_BLOCK_PATTERNS = ['ECONNRESET', 'connection reset', 'socket hang up'];
+const SNI_BLOCK_PATTERNS = ['ECONNRESET', 'connection reset', 'socket hang up', 'socket disconnected before secure TLS'];
 
 async function checkKoreaProbes(domain: string): Promise<GlobalpingProbeResult[]> {
   // Create 3 measurements in parallel: