frontend: fix wallet push silently skipping in form dialogs

The "send to wallet" step in the appointment/invoice/prescription/
patient-edit/scribe dialogs was gated on sync.linked, which resolves
asynchronously via getWalletLink in an effect. A fast save (or a
transient failure that collapsed it to false) left a wallet-linked
patient looking unlinked, so the dialog took the else branch and pushed
nothing.

- Add ensureLinked() to useWalletSync: awaits getWalletLink at submit and
  returns the resolved status. Each of the 5 dialogs now awaits it before
  deciding whether to show the wallet step.
- Harden push() to drop empty/whitespace changes (the backend 400s on an
  empty change set) and add a translated summaryFallback so the step
  never sends an empty summary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/components/invoices/invoice-form-dialog.tsx

@@ -273,7 +273,7 @@ export function InvoiceFormDialog({
             })
           : await createInvoice(payload);
       onSaved(saved);
-      if (sync.linked) {
+      if (await sync.ensureLinked()) {
         setWalletSummary(
           mode === "edit"
             ? t("walletSync.summary.invoiceUpdated", { number: saved.number })