fix(terminal): bound websocket input frames

cmd/dashboard/controller/terminal_input_limit_test.go

@@ -0,0 +1,9 @@
+package controller
+
+import "testing"
+
+func TestTerminalWebSocketInputLimitAllowsBoundedPaste(t *testing.T) {
+	if terminalWebSocketInputLimit != 512*1024+64 {
+		t.Fatalf("terminal WebSocket input limit = %d, want 512 KiB plus control-byte allowance", terminalWebSocketInputLimit)
+	}
+}