A Raspberry Pi 4 with no screen attached waits for one, and other improvements

Fixes

• A Raspberry Pi 4 that starts with no display attached waits for one instead of crash-looping with no screens available. When a screen is connected or powered on, the viewer starts by itself. This is the Pi 4 counterpart of the wait that Pi 2 and Pi 3 got in 2026.05.2.

For contributors

• bin/balena_fleet_maintenance.py moves devices on the 5 official screenly_ose/anthias-* balena fleets forward a batch at a time. For each device it removes the pin to a release, so the device follows the fleet's latest stable release, and starts an update to the latest balenaOS. It does a dry run unless you pass --apply, skips offline devices, takes 1% of the remaining devices per run unless you set --percent or --count, records finished devices in .balena-maint-state.tsv so the next run skips them, and moves on when a device fails.