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.

docs/USAGE.md

@@ -18,8 +18,8 @@ tuimble --host mumble.example.com --user myname
 
 ## Push-to-Talk Modes
 
-- **hold** — hold key to transmit, release to stop (default)
-- **toggle** — press to start, press again to stop
+- **toggle** — press to start, press again to stop (default)
+- **hold** — hold key to transmit, release to stop (requires evdev)
 
 ## Configuration