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>

res/lang/zh.json

@@ -874,7 +874,14 @@
     "peers_upper": "节点",
     "peers_version": "版本",
     "pending": "待处理",
+    "pin_change_desc": "更改您的解锁 PIN 码。您需要当前 PIN 码和新 PIN 码。",
+    "pin_confirm_new_label": "确认新 PIN 码：",
+    "pin_current_label": "当前 PIN 码：",
+    "pin_new_label": "新 PIN 码（4-8 位）：",
     "pin_not_set": "未设置 PIN。使用密码解锁。",
+    "pin_remove_desc": "输入当前 PIN 码以确认移除。解锁时您需要使用完整的密码短语。",
+    "pin_setup_desc": "设置一个 4-8 位 PIN 码以快速解锁钱包。您的钱包密码短语将使用此 PIN 码加密并存储在本地。",
+    "pin_wallet_passphrase": "钱包密码短语：",
     "ping": "延迟",
     "portfolio_add_entry": "添加条目",
     "portfolio_add_to": "添加到投资组合",