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/rpc/rpc_client.h

@@ -270,8 +270,10 @@ private:
     // hold curl_mutex_ and have verified impl_->curl.
     std::string performCall(const std::string& method, const json& params, long& httpCodeOut);
     // Centralizes the HTTP-code check and JSON error->RpcError extraction, returning
-    // response["result"] on success.
-    static json parseRpcResult(long httpCode, const std::string& body);
+    // response["result"] on success. When scrubSource is true (secret-bearing calls), the intermediate
+    // parse tree's string values are zeroed before it is discarded — parseRpcResult returns a COPY of
+    // the result node, so its own tree would otherwise free the secret un-wiped (B7).
+    static json parseRpcResult(long httpCode, const std::string& body, bool scrubSource = false);
 
     // Splits a UnifiedCallback into the (Callback, ErrorCallback) pair used by doRPC.
     static std::pair<Callback, ErrorCallback> splitUnified(UnifiedCallback cb);