feat(send): migrate the Shield/Merge modal to the reference design

Presentation-only migration of ShieldDialog::render (Shield coinbase / Merge funds)
— a fund-moving modal:
- struct-form BeginOverlayDialog(OverlayDialogSpec{BlurFloat, cardWidth win.width,
  idSuffix shielddialog}) replacing the legacy positional overload
- drop the description divider and the form->footer divider (keep the post-submit
  operation-id section divider)
- 3 StyledButton -> TactileButton (submit / cancel / check-status)
- TR() the hardcoded "No shielded (z) address yet …" note -> new shield_no_zaddr_hint
  key, 8 langs (em-dash preserved); the "DRGX" ticker stays hardcoded per convention

The fund path is byte-for-byte unchanged: the can_submit guard, fee/utxo clamps, the
"ANY_TADDR" setup, the z_shieldcoinbase / z_mergetoaddress worker->post calls + their
opid/status/Notifications/trackOperation callbacks, and the BeginDisabled/tooltip logic.

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

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

src/util/i18n.cpp

@@ -1858,6 +1858,7 @@ void I18n::loadBuiltinEnglish()
         strings_["shield_max_utxos"] = "Max UTXOs per operation";
         strings_["shield_merge_done"] = "Shield/merge completed!";
         strings_["shield_select_z"] = "Select z-address...";
+        strings_["shield_no_zaddr_hint"] = "No shielded (z) address yet — create one on the Receive tab first.";
         strings_["shield_started"] = "Shield operation started";
         strings_["shield_title"] = "Shield Coinbase Rewards";
         strings_["shield_to_address"] = "To Address (Shielded):";