ace_direct mode (no toolhead buffer) only kept the first reported ACE
unit and silently dropped any others. Affects e.g. the Kobra S1 with two
ACE Pro units - the dashboard and OrcaSlicer sync only ever saw 4 of 8
slots. Global slot index is now box_id * 4 + local slot across all
units, matching the existing //4-%4 fallback in _global_to_box_slot.
Single-ACE (Kobra X) behavior is unchanged (unit id 0 -> same indices
as before).
feat(store): multi-select + bulk delete in the GCode browser (Issue #94)
Checkbox on every file card enters select mode; clicking anywhere on a
selected-mode card toggles it, existing per-file actions keep working via
stopPropagation. 'Select All' only affects the currently filtered/visible
files. Bulk delete is N parallel calls to the existing single-file DELETE
endpoint (no new backend route) with one confirmation dialog.
