feat(install): updates refresh systemd units; new installs run the newest release (#729)

Updates that move HEAD now install changed systemd units through a root-owned helper (/usr/local/sbin/ledmatrix-refresh-units, two literal sudo lines), with a backup restored on rollback; a refresh that fails part-way puts the old units back. Devices without the new sudo rule keep updating and are told to re-run the installer once. The one-shot installer now checks out the newest vX.Y.Z release (LEDMATRIX_CHANNEL=beta keeps main) and never moves an existing checkout backwards.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/auto_update.py

@@ -603,6 +603,9 @@ class AutoUpdater:
             'release': info.get('release') if new_head == info.get('upstream_head') else None,
             'display_was_active': display_was_active,
             'dependency_failures': list(core.get('dependency_failures') or []),
+            # The update installed new systemd units: a rollback puts the
+            # previous ones back (ledmatrix-refresh-units --restore).
+            'units_refreshed': (core.get('unit_refresh') or {}).get('status') == 'refreshed',
             'created_at': self.clock(),
         }