fix(rpc,ui): close the last secret residues found by the final-gate review

Two verified medium leaks in the B7 scrub chain:

- parseRpcResult parses the body into a local `response` tree and returns a COPY
  of response["result"] (operator[] yields an lvalue ref, so the by-value return
  copy-constructs). The local tree — holding its own heap copy of the secret — was
  then freed without zeroing, so callSecret/callSecretString still left one
  un-scrubbed copy. Add scrubJsonSecrets() (recursive string zero) and a
  scrubSource flag; the secret paths opt in, wiping the tree before it frees. The
  secret export chain is now fully covered: raw body → parse tree → result copy →
  caller-owned string.

- key_export_dialog cleared s_key with plain std::string::clear() on the Close
  button, the scrim/Esc dismiss path, and the QR cache (s_qr_cached) — leaving the
  displayed private/spending key in freed heap on the ordinary close paths. Route
  all three through wallet::secureWipeLiteSecret (zero-then-clear), matching
  show()/hide().

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

src/ui/windows/key_export_dialog.cpp

@@ -38,7 +38,7 @@ std::string KeyExportDialog::s_error;
 void KeyExportDialog::releaseQr()
 {
     if (s_qr_tex) { FreeQRTexture(s_qr_tex); s_qr_tex = 0; }
-    s_qr_cached.clear();
+    wallet::secureWipeLiteSecret(s_qr_cached);   // held a plaintext copy of the key for the QR
     s_show_qr = false;
 }
 
@@ -318,8 +318,8 @@ void KeyExportDialog::render(App* app)
 
         if (material::TactileButton(TR("close"), ImVec2(button_width, 0), S.resolveFont(closeBtn.font))) {
             s_open = false;
-            // Clear sensitive data
-            s_key.clear();
+            // Zero the secret, don't just drop the buffer (leaves the key in freed heap otherwise).
+            wallet::secureWipeLiteSecret(s_key);
             s_show_key = false;
             releaseQr();
         }
@@ -327,9 +327,9 @@ void KeyExportDialog::render(App* app)
         material::EndOverlayDialog();
     }
 
-    // Dialog dismissed any other way (scrim click / Esc): drop the key + its QR texture.
+    // Dialog dismissed any other way (scrim click / Esc): wipe the key + its QR texture.
     if (!s_open) {
-        if (!s_key.empty()) s_key.clear();
+        wallet::secureWipeLiteSecret(s_key);
         s_show_key = false;
         releaseQr();
     }