fix: issue invoice on demand so view-invoice buttons actually work

The admin SPA never called POST /billing/invoices, so every session had
invoice_uuid = null and the view-invoice buttons on the patient services
tab (and MyPatients visit modal) were permanently disabled. Add a shared
useIssueInvoice hook (idempotent create + finalize when draft) and wire
it into:
- DetailsStep: the wizard's final step now really issues the invoice
- SessionServiceCard / PatientDetailPage: clicking view-invoice on a
  session without an invoice issues it first, then opens the summary
- MyPatientsPage visit modal: same, replacing the dead disabled button

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

assets/admin/components/SessionServiceCard.tsx

@@ -45,11 +45,14 @@ function Row({ label, value, valueStyle }: { label: string; value: string; value
  * pixel-for-pixel from tauri files/services/ServiceCard. Paid cards show
  * «مشاهده فاکتور», unpaid ones «تکمیل پرداخت».
  */
-export default function SessionServiceCard({ session, onSettle, onViewInvoice, settling }: {
+export default function SessionServiceCard({ session, onSettle, onViewInvoice, settling, issuing }: {
   session: SessionCardData;
   onSettle: (uuid: string) => void;
-  onViewInvoice: (invoiceUuid: string) => void;
+  /** کل session پاس می‌شود؛ اگر invoice_uuid نداشت، caller فاکتور را می‌سازد. */
+  onViewInvoice: (session: SessionCardData) => void;
   settling?: boolean;
+  /** true وقتی صدور فاکتور همین لحظه در جریان است (دکمه قفل می‌شود). */
+  issuing?: boolean;
 }) {
   const paid = !!session.is_paid;
   const names = (session.services ?? []).map((s) => s.service_name || s.name).filter(Boolean) as string[];
@@ -94,11 +97,11 @@ export default function SessionServiceCard({ session, onSettle, onViewInvoice, s
         {paid ? (
           <button
             type="button"
-            disabled={!session.invoice_uuid}
-            onClick={() => session.invoice_uuid && onViewInvoice(session.invoice_uuid)}
+            disabled={issuing}
+            onClick={() => onViewInvoice(session)}
             style={{ marginTop: 8, height: 40, width: '100%', borderRadius: 4, fontSize: 13, fontWeight: 500, cursor: 'pointer', color: '#4F46E5', background: 'transparent', border: '1px solid #5559ce' }}
           >
-            مشاهده فاکتور
+            {issuing ? 'در حال صدور…' : 'مشاهده فاکتور'}
           </button>
         ) : (
           <button