ptt: change default key from space to f4

Space conflicts with chat input. F4 is unused by default
in most terminal emulators and won't interfere with typing.

tests/test_config.py

@@ -19,5 +19,5 @@ def test_server_config():
 
 def test_ptt_config_defaults():
     ptt = PttConfig()
-    assert ptt.key == "space"
+    assert ptt.key == "f4"
     assert ptt.backend == "auto"