Add turn_on/turn_off/toggle support

- Add API_TURN_ON, API_TURN_OFF, API_TOGGLE constants
- Add turn_on(), turn_off(), toggle() methods to MediaServerClient
- Implement async_turn_on, async_turn_off, async_toggle in media player
- Add TURN_ON and TURN_OFF to supported features
- Update README with turn on/off/toggle documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -12,6 +12,7 @@ A Home Assistant custom integration that allows you to control a remote PC's med
 - Seek support with smooth timeline updates
 - Displays current track info (title, artist, album, artwork)
 - Real-time updates via WebSocket (with HTTP polling fallback)
+- **Turn on/off/toggle support** - Execute custom actions (e.g., lock screen on turn off)
 - **Script buttons** - Execute pre-defined scripts (shutdown, restart, lock, sleep, etc.)
 - Configurable via Home Assistant UI
 
@@ -69,6 +70,31 @@ A full-featured media player entity with:
 - Volume control and mute
 - Seek functionality
 - Current track information
+- Turn on/off/toggle actions (execute server-side callbacks)
+
+### Turn On/Off/Toggle
+
+The media player supports `media_player.turn_on`, `media_player.turn_off`, and `media_player.toggle` actions. These execute optional callbacks configured on the Media Server (e.g., lock screen on turn off).
+
+Configure callbacks in Media Server's `config.yaml`:
+
+```yaml
+callbacks:
+  on_turn_on:
+    command: "echo PC turned on"
+    timeout: 10
+    shell: true
+
+  on_turn_off:
+    command: "rundll32.exe user32.dll,LockWorkStation"
+    timeout: 5
+    shell: true
+
+  on_toggle:
+    command: "echo Toggle triggered"
+    timeout: 10
+    shell: true
+```
 
 ### Script Button Entities