feat(settings): gate debug options behind a confirmation (+ re-auth if secured)

Revealing the DEBUG OPTIONS dropdown now opens a confirmation modal with a
warning instead of expanding directly. If the wallet is secured, it also requires
re-authentication before the options appear:
- a quick-unlock PIN → verified against the vault (Argon2id derive, off-thread,
  no wallet side effects);
- otherwise an encrypted wallet → the passphrase, verified via walletpassphrase.
Unsecured wallets just confirm. Collapsing needs no gate; each expand re-gates.

App gains debugGateRequiresAuth() + verifyDebugCredential() (cb on the main
thread; secrets wiped). Adds a modal-debug-gate sweep surface. Verified 100% +
150%, dark + light.

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

src/app.h

@@ -495,6 +495,12 @@ public:
     void showPinRemoveDialog() { show_pin_remove_ = true; pin_status_.clear(); }
     bool hasPinVault() const;
 
+    // Debug-options gate: does revealing the debug dropdown need re-authentication (a PIN vault or
+    // an encrypted wallet)? And verify the entered PIN/passphrase — cb(ok) is invoked on the main
+    // thread (PIN via the vault, else the wallet passphrase via RPC).
+    bool debugGateRequiresAuth() const;
+    void verifyDebugCredential(const std::string& secret, std::function<void(bool)> cb);
+
     /// @brief Check if RPC worker has queued results waiting to be processed
     bool hasPendingRPCResults() const;
     bool hasTransactionSendProgress() const { return send_progress_active_ || send_submissions_in_flight_ > 0 || !pending_opids_.empty(); }