feat(send): migrate the Confirm-Send modal to the reference design

Presentation-only migration of RenderSendConfirmPopup (the confirm-before-broadcast
popup) — a fund-moving modal:
- struct-form BeginOverlayDialog(OverlayDialogSpec{BlurFloat, cardWidth popupW,
  idSuffix confirmsend}) replacing the legacy positional overload; p_open stays
  nullptr (the popup keeps its own Escape + Cancel handling)
- TR() the one hardcoded status string in the synchronous live-revalidation guard
  ("Cannot send now …") -> new send_cannot_send_now key, 8 langs (em-dash preserved)

The fund path is byte-for-byte unchanged: the live re-validation guard, the
app->sendTransaction call + its success/failure callback, the lite-locked
requestLiteUnlock branch, the s_sending/s_show_confirm state, the Escape handler,
and the FROM/TO cards + fee-tier/summary rendering.

Verified: build + ctest + hygiene clean; a 2-lens adversarial review
(fund-path-untouched / ui-i18n) returned zero findings, with the fund-path lens
proving the send path diffs byte-for-byte identical to HEAD.

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

res/lang/de.json

@@ -1084,6 +1084,7 @@
     "send_amount": "Betrag",
     "send_amount_details": "BETRAGSDETAILS",
     "send_amount_upper": "BETRAG",
+    "send_cannot_send_now": "Senden derzeit nicht möglich — prüfen Sie Verbindung, Synchronisierung und verfügbares Guthaben.",
     "send_clear_fields": "Alle Formularfelder leeren?",
     "send_contacts_button": "Aus Kontakten auswählen",
     "send_copy_error": "Fehler kopieren",