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>

src/util/i18n.cpp

@@ -1837,6 +1837,7 @@ void I18n::loadBuiltinEnglish()
         strings_["send_total"] = "Total";
         strings_["send_tx_failed"] = "Transaction failed";
         strings_["send_tx_sent"] = "Transaction sent!";
+        strings_["send_cannot_send_now"] = "Cannot send now — check connection, sync, and available balance.";
         strings_["send_tx_success"] = "Transaction sent successfully!";
         strings_["send_status_unconfirmed"] = "Transaction status could not be confirmed";
         strings_["send_err_needs_rescan"] = "Your wallet's shielded note data is out of date with the blockchain (this happens after a bootstrap or reindex). Run a full rescan via Settings -> Rescan Blockchain and let it finish completely, then try sending again.";