feat(settings): redesign the PIN setup/change/remove modals onto reference design

Migrate the three PIN dialogs in App::renderPinDialogs (Set PIN / Change PIN /
Remove PIN) onto the settings-modal reference design — presentation-only, plus a
deliberate secret-hygiene addition:

- struct-form BeginOverlayDialog(OverlayDialogSpec{BlurFloat, cardWidth 420/420/400,
  idSuffix pinsetup/pinchange/pinremove}) replacing the legacy positional overload
- drop each dialog's redundant post-description Separator
- every rendered-chrome string -> TR(): titles/buttons reuse settings_set_pin /
  settings_change_pin / settings_remove_pin, the Set-PIN confirm reuses
  wiz_pin_confirm; 7 new pin_* keys added to the English source + all 8
  res/lang/*.json additively; ImGui::Text -> TextUnformatted for labels
- HARDENING: wipe the passphrase/PIN input buffers on BlurFloat dismiss
  (X/Esc/outside-click) for all three dialogs. Set PIN's wallet-passphrase buffer
  (pin_passphrase_buf_) was previously left resident on dismiss; it is now zeroed.

The vault/RPC path is byte-for-byte unchanged: the submit-time memsets, the
walletpassphrase/walletlock RPCs, vault_->store/changePin/retrieve/removeVault,
SecureVault::secureZero, and setPinEnabled/save are untouched. The pin_status_ and
toast strings inside the worker callbacks are left in English (a separate
service-layer i18n pass), to avoid editing the secret path.

Verified: build + ctest + hygiene clean; i18n complete in 8 langs with no format
specifiers; a 3-lens adversarial review (secret-hygiene / presentation-only /
ui-i18n) returned zero findings, with the presentation lens proving the vault path
diffs byte-for-byte identical against HEAD.

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

src/util/i18n.cpp

@@ -537,6 +537,14 @@ void I18n::loadBuiltinEnglish()
         strings_["decrypt_success_desc"] = "Your wallet is now unencrypted. A backup of the encrypted wallet was saved as wallet.dat.encrypted.bak in your data directory.";
         strings_["decrypt_error_title"] = "Decryption failed";
         strings_["try_again"] = "Try Again";
+        // --- PIN setup / change / remove dialogs (settings) ---
+        strings_["pin_setup_desc"] = "Set a 4-8 digit PIN for quick wallet unlock. Your wallet passphrase will be encrypted with this PIN and stored locally.";
+        strings_["pin_wallet_passphrase"] = "Wallet Passphrase:";
+        strings_["pin_new_label"] = "New PIN (4-8 digits):";
+        strings_["pin_change_desc"] = "Change your unlock PIN. You need your current PIN and a new PIN.";
+        strings_["pin_current_label"] = "Current PIN:";
+        strings_["pin_confirm_new_label"] = "Confirm New PIN:";
+        strings_["pin_remove_desc"] = "Enter your current PIN to confirm removal. You will need to use your full passphrase to unlock.";
         strings_["wiz_confirm"] = "Confirm:";
         strings_["wiz_strength_weak"] = "Weak";
         strings_["wiz_strength_fair"] = "Fair";