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/pages/SessionPaymentPage.tsx

@@ -98,7 +98,7 @@ export default function SessionPaymentPage() {
               onCancel={() => nav(-1)}
             />
           ) : (
-            <DetailsStep session={session} onBack={() => setActiveStep(0)} onFinish={finish} />
+            <DetailsStep session={session} recordUuid={recordUuid} onBack={() => setActiveStep(0)} onFinish={finish} />
           )}
         </div>
       </div>