fix(ui): unify checkbox size — drop size="small" on raw checkboxes

The kit CheckboxField (used on most pages) renders MUI's default medium checkbox, but four raw <Checkbox> usages forced size="small": OfferDetail's 'V ceně' table cell and 'Účtovat DPH' form checkbox, and the Settings permission-matrix module + per-permission checkboxes. They looked undersized next to the rest of the page. Removing size="small" makes every checkbox the same standard size app-wide.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/pages/OfferDetail.tsx

@@ -306,7 +306,6 @@ function SortableItemRow({
           checked={item.is_included_in_total}
           onChange={(e) => onUpdate("is_included_in_total", e.target.checked)}
           disabled={readOnly}
-          size="small"
         />
       </TableCell>
       <TableCell
@@ -1312,7 +1311,6 @@ export default function OfferDetail() {
                   checked={form.apply_vat}
                   onChange={(e) => updateForm("apply_vat", e.target.checked)}
                   disabled={readOnly}
-                  size="small"
                 />
                 <Box component="span">Účtovat DPH</Box>
               </Box>