Add aggressive HDMI power control to prevent display shutdown

Enhanced Raspberry Pi HDMI power management:

Boot Configuration (/boot/config.txt):
- hdmi_blanking=0 - Disable HDMI blanking
- hdmi_force_hotplug=1 - Force HDMI mode always on
- hdmi_ignore_cec_init=1 - Disable HDMI CEC interference
- hdmi_ignore_edid=0xa5000080 - Disable HDMI sleep mode

New systemd services:
- hdmi-poweron.service - Runs tvservice -p every 30 seconds
- screen-keepalive.service - Aggressive display keep-alive script

Display keep-alive script (.display-keepalive.sh):
- tvservice -p - Force HDMI to stay powered
- xset commands - Keep X11 screensaver off
- xdotool - Mouse movement for activity detection
- xrandr - Disable monitor power profile saving
- 30-second loop for continuous power signaling

Enhanced app-level control (main.py):
- DISPLAY=:0 environment variable for X11 commands
- tvservice integration in signal_screen_activity()
- xrandr power profile management
- More aggressive DPMS disabling

This addresses HDMI being powered down at multiple levels:
1. Firmware level (boot config)
2. System level (systemd services)
3. X11 level (xset, xdotool, xrandr)
4. App level (continuous signaling)

HDMI output should now remain powered throughout playback.

.player_heartbeat

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