Add CRM order queue: live sync, view modal, receipt tabs, delete

Wires the CRM (exoCRM GetOrders) into the dashboard as a locally
persisted order queue instead of the previous static mockup:

- CrmClient Protocol + ExoCrmClient/StubCrmClient for the CRM's
  signed JSON-RPC API
- Order model + migration, synced from CRM on each queue view;
  soft-deleted orders stay hidden across re-syncs
- GET/DELETE /api/v1/orders with "no receipt"/"receipt issued" tabs
  (the latter is empty until Checkbox fiscalization lands)
- Dashboard: real order list, item-detail modal, tab switcher,
  one-click delete

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/Modal.css

@@ -0,0 +1,21 @@
+.modal-overlay {
+  position: fixed;
+  inset: 0;
+  background: rgba(0, 0, 0, 0.45);
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 24px;
+  z-index: 100;
+}
+
+.modal-content {
+  background: var(--color-surface);
+  border: 1px solid var(--color-border);
+  border-radius: 12px;
+  box-shadow: var(--shadow-card);
+  max-width: 640px;
+  width: 100%;
+  max-height: 90vh;
+  overflow-y: auto;
+}