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/ui/pages/settings_page.h

@@ -15,5 +15,9 @@ namespace ui {
  */
 void RenderSettingsPage(App* app);
 
+// Sweep-only: force the debug-options confirmation gate open/closed so the offline UI sweep can
+// capture it (it is otherwise opened only by clicking the DEBUG OPTIONS header).
+void SweepOpenDebugGate(bool open);
+
 } // namespace ui
 } // namespace dragonx