fix: ptt auto-detect falls back to toggle mode

Textual does not expose key-release events, so KittyPtt
hold-mode never received key_up and stayed transmitting.
Auto-detect now tries evdev first, then falls back to
toggle (press-on/press-off). Default mode changed to toggle.

tests/test_config.py

@@ -8,7 +8,7 @@ def test_default_config():
     assert cfg.server.host == "localhost"
     assert cfg.server.port == 64738
     assert cfg.audio.sample_rate == 48000
-    assert cfg.ptt.mode == "hold"
+    assert cfg.ptt.mode == "toggle"
 
 
 def test_server_config():