The previous commit fixed battery_mv in the STATS_TYPE_CORE frame, but that is
not the path companion apps use to display battery. CMD_GET_BATT_AND_STORAGE
goes through `CompanionFrameServer._get_batt_and_storage()`, an openhop_core
extension hook documented as "Default: all zeros" that the repeater never
overrode — so apps showed 0% regardless of what any sensor reported.
Override it in the repeater's frame server: battery from the same sensor lookup
added for core stats, storage from the filesystem holding the configured
storage_dir. Both are wired in from main.py alongside stats_getter.
Found by deploying the previous fix and observing that Home Assistant showed the
modem battery correctly while the phone app still showed 0% — two different
paths, only one of which had been fixed.