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/fr.json

@@ -874,7 +874,14 @@
     "peers_upper": "PAIRS",
     "peers_version": "Version",
     "pending": "En attente",
+    "pin_change_desc": "Modifiez votre code PIN de déverrouillage. Vous avez besoin de votre code PIN actuel et d'un nouveau code PIN.",
+    "pin_confirm_new_label": "Confirmer le nouveau code PIN :",
+    "pin_current_label": "Code PIN actuel :",
+    "pin_new_label": "Nouveau code PIN (4 à 8 chiffres) :",
     "pin_not_set": "PIN non défini. Utilisez le mot de passe pour déverrouiller.",
+    "pin_remove_desc": "Saisissez votre code PIN actuel pour confirmer la suppression. Vous devrez utiliser votre phrase secrète complète pour déverrouiller.",
+    "pin_setup_desc": "Définissez un code PIN de 4 à 8 chiffres pour déverrouiller rapidement votre portefeuille. Votre phrase secrète sera chiffrée avec ce code PIN et stockée localement.",
+    "pin_wallet_passphrase": "Phrase secrète du portefeuille :",
     "ping": "Ping",
     "portfolio_add_entry": "Ajouter une entrée",
     "portfolio_add_to": "Ajouter au portefeuille",