fix(tss-party): increase stdin buffer for large TSS messages (~200KB)

Default bufio.Scanner buffer (64KB) was too small for TSS keygen messages
which can reach ~178KB. Increased to 1MB to handle large messages.

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

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

backend/mpc-system/services/service-party-app/tss-party/main.go

@@ -223,6 +223,9 @@ func executeKeygen(
 	go func() {
 		defer inWg.Done()
 		scanner := bufio.NewScanner(os.Stdin)
+		// Increase buffer for large messages (TSS messages can be ~200KB)
+		buf := make([]byte, 1024*1024)
+		scanner.Buffer(buf, len(buf))
 		for scanner.Scan() {
 			select {
 			case <-ctx.Done():