Filter unsupported DEC private mode 7727 from terminal output

Shells (e.g. via Tera Term conventions) emit CSI ?7727h/l to toggle
Application Escape Key mode, which ghostty-web does not implement.
This produces noisy console warnings in the browser.

Strip these sequences server-side in the output pipeline (both
TerminalSession and DockerExecSession) before they reach the client,
using the same pattern as the existing DA response filter.

webterm/docker_exec_session.go

@@ -147,6 +147,7 @@ func (s *DockerExecSession) handleOutput(data []byte) {
 	tracker := s.tracker
 	connector := s.connector
 	s.mu.Unlock()
+	filtered = FilterUnsupportedModes(filtered)
 	dispatchSessionOutput(filtered, tracker, s.replay, connector)
 }
 

webterm/normalize.go

@@ -6,8 +6,9 @@ import (
 )
 
 var (
-	daResponsePattern = regexp.MustCompile(`\x1b\[[?>=][\d;]*c`)
-	daPartialPattern  = regexp.MustCompile(`\x1b(?:\[(?:[?>=][\d;]*)?)?$`)
+	daResponsePattern       = regexp.MustCompile(`\x1b\[[?>=][\d;]*c`)
+	daPartialPattern        = regexp.MustCompile(`\x1b(?:\[(?:[?>=][\d;]*)?)?$`)
+	unsupportedModePattern  = regexp.MustCompile(`\x1b\[\?7727[hl]`)
 )
 
 func FilterDASequences(data []byte, escapeBuffer []byte) ([]byte, []byte) {
@@ -28,3 +29,12 @@ func FilterDASequences(data []byte, escapeBuffer []byte) ([]byte, []byte) {
 	}
 	return filtered, nil
 }
+
+// FilterUnsupportedModes strips DEC private mode sequences that ghostty-web
+// does not implement, preventing noisy console warnings in the browser.
+func FilterUnsupportedModes(data []byte) []byte {
+	if !bytes.Contains(data, []byte("\x1b[?7727")) {
+		return data
+	}
+	return unsupportedModePattern.ReplaceAll(data, nil)
+}

webterm/terminal_session.go

@@ -136,6 +136,7 @@ func (s *TerminalSession) handleOutput(data []byte) {
 	tracker := s.tracker
 	connector := s.connector
 	s.mu.Unlock()
+	filtered = FilterUnsupportedModes(filtered)
 	dispatchSessionOutput(filtered, tracker, s.replay, connector)
 }