Add Checkbox ETTN receipts for Nova Poshta COD waybills

Cashier creates an ETTN receipt in Checkbox bound to the TTN with payment
control; Checkbox fiscalizes it itself when the parcel is paid for.

- cash_registers (Fernet-encrypted license key / PIN) and receipts tables
- Checkbox HTTP client + stub (ETTN does not work on test registers)
- two-phase create via ARQ job, timeout reconciliation, cron status polling
- /receipts and /cash-registers API, audit records
- dashboard: per-order and bulk create, prepayment, cancel; cash registers page

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

frontend/src/pages/DashboardPage.css

@@ -325,3 +325,68 @@
   color: var(--color-text-muted);
   padding: 32px 0;
 }
+
+.orders-toolbar-actions {
+  display: flex;
+  gap: 8px;
+  align-items: center;
+}
+
+.orders-link-btn {
+  border: 1px solid var(--color-border);
+  border-radius: 8px;
+  padding: 9px 14px;
+  font-size: 14px;
+  color: var(--color-text);
+  text-decoration: none;
+}
+
+.orders-prepayment-input--invalid {
+  border-color: #dc2626;
+}
+
+.orders-notice {
+  display: flex;
+  justify-content: space-between;
+  align-items: flex-start;
+  gap: 12px;
+  margin-bottom: 12px;
+  padding: 10px 14px;
+  border-radius: 8px;
+  font-size: 14px;
+}
+
+.orders-notice ul {
+  margin: 0;
+  padding-left: 18px;
+}
+
+.orders-notice--ok {
+  background: #dcfce7;
+  color: #166534;
+}
+
+.orders-notice--error {
+  background: #fee2e2;
+  color: #991b1b;
+}
+
+.orders-notice-close {
+  border: none;
+  background: none;
+  color: inherit;
+  font-size: 18px;
+  cursor: pointer;
+}
+
+@media (prefers-color-scheme: dark) {
+  .orders-notice--ok {
+    background: rgba(22, 163, 74, 0.18);
+    color: #86efac;
+  }
+
+  .orders-notice--error {
+    background: rgba(220, 38, 38, 0.18);
+    color: #fca5a5;
+  }
+}