fix(tss-wasm): correct signing rounds from 6 to 9

GG20 signing protocol has 9 rounds, not 6. This aligns WASM with
Electron (tss-party/main.go:717) and Android (tsslib/tsslib.go:477).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/tss-wasm/main.go

@@ -403,9 +403,9 @@ func stopSession(this js.Value, args []js.Value) interface{} {
 
 // handleOutgoingMessages processes messages from the TSS protocol
 func (s *TSSSession) handleOutgoingMessages() {
-	totalRounds := 4
+	totalRounds := 4 // GG20 keygen has 4 rounds
 	if !s.IsKeygen {
-		totalRounds = 6 // Signing has 6 rounds
+		totalRounds = 9 // GG20 signing has 9 rounds (matching Electron and Android)
 	}
 
 	for {