Add screen activity signaler to prevent display sleep

- Added signal_screen_activity() method to SignagePlayer class
- Runs every 20 seconds automatically
- Also triggered on any touch/user input events

Multiple methods used to keep display awake:
- xset s reset - Resets screensaver timer
- xset dpms force on - Forces display on
- xdotool - Subtle mouse movement to trigger activity

This complements the system-level power management:
- Works alongside display power management settings
- Non-blocking and non-critical (fails gracefully)
- Signals every 20 seconds + on user input
- Prevents display from sleeping during playback

Screen should now remain active throughout media playback.

.player_heartbeat

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