security: disable OSC52 by default

Disable OSC52 clipboard access by default for security reasons:
- Change default-config.json: allow_osc52 from true to false
- Users can still enable it manually if needed
- Prevents potential clipboard data leakage by default

Rationale:
- OSC52 allows remote servers to access clipboard
- Should be explicitly enabled by users who need it
- Follows security best practice: deny by default

Author: dzw1995

src/assets/other/default-config.json

@@ -340,7 +340,7 @@
                             "key": "allow_osc52",
                             "type": "checkbox",
                             "text": "Allow OSC52 clipboard access",
-                            "default": true
+                            "default": false
                         },
                         {
                             "key": "set_cursor_position",