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>

res/lang/fr.json

@@ -1223,6 +1223,7 @@
     "shield_in_progress": "Opération en cours...",
     "shield_max_utxos": "UTXOs max par opération",
     "shield_merge_done": "Blindage/fusion terminé !",
+    "shield_no_zaddr_hint": "Aucune adresse blindée (z) pour l'instant — créez-en une d'abord dans l'onglet Recevoir.",
     "shield_op_failed": "Échec de l'opération : ",
     "shield_op_submitted": "Opération soumise : ",
     "shield_operation_id": "ID d'opération : %s",