fix(plan): drop duplicate 'Zrušit' in the view-mode cell modal

The read-only plan-cell detail modal (view mode) already has a "Zavřít"
submit button, but the kit Modal also rendered its built-in "Zrušit"
cancel — two buttons that do the same thing. Pass hideCancel so only
"Zavřít" remains (same fix as the category-management modal).

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

src/admin/components/PlanCellModal.tsx

@@ -479,6 +479,7 @@ function ViewModal(props: Props) {
       onClose={onClose}
       onSubmit={onClose}
       submitText="Zavřít"
+      hideCancel
     >
       <Typography sx={{ mb: 1 }}>
         <strong>Datum:</strong> {formatDate(c.shift_date)}