Add autostart functionality and power management for Raspberry Pi

- Enhanced install.sh with comprehensive autostart workflow:
  * XDG autostart entry (desktop environment)
  * systemd user service (most reliable)
  * LXDE autostart support (Raspberry Pi OS)
  * Cron fallback (@reboot)
  * Terminal mode enabled for debugging

- Added Raspberry Pi power management features:
  * Disable HDMI screen blanking
  * Prevent CPU power saving (performance mode)
  * Disable system sleep/suspend
  * X11 screensaver disabled
  * Display power management (DPMS) disabled

- Fixed sudo compatibility:
  * Properly detects actual user when run with sudo
  * Correct file ownership for user configs
  * systemctl --user works correctly

- Player launches in terminal for error visibility
- Autostart configured to use start.sh (watchdog with auto-restart)

playlists/server_playlist.json

@@ -0,0 +1,21 @@
+{
+  "count": 2,
+  "player_id": 1,
+  "player_name": "TVacasa",
+  "playlist": [
+    {
+      "file_name": "2026efvev-1428673176.jpg",
+      "url": "media/2026efvev-1428673176.jpg",
+      "duration": 50,
+      "edit_on_player": false
+    },
+    {
+      "file_name": "4k1.jpg",
+      "url": "media/4k1.jpg",
+      "duration": 30,
+      "edit_on_player": false
+    }
+  ],
+  "playlist_id": 1,
+  "playlist_version": 29
+}