security(hygiene): best-effort scrubbing of residual in-memory secret copies (Tier 4)

Defense-in-depth against a local attacker / core dump reading un-zeroed heap
copies of secrets (threat model is local, not remote). Best-effort by nature —
std::string SSO/reallocation and ImGui buffer growth can still strand copies —
so these shrink the exposure window rather than guarantee absence:
- rpc_client performCall: a scope guard now sodium_memzeroes the request body +
  payload tree (walletpassphrase / *_importkey args) after the call and detaches
  CURLOPT_POSTFIELDS. Request content sent to curl is unchanged.
- New ui::scrubImGuiInputTextState() zeroes ImGui's retained InputText buffers
  (live/revert/callback/deactivated) — the app memzeroed its own char[]s but
  ImGui kept copies. Called from wipeSecrets()/wipeDisplayedSecrets(). PINNED to
  ImGui 1.92.6 via a #error guard so a vendored bump forces a re-audit (the stb
  undo buffer is an incomplete type in the header, so it's left as a noted gap).
- Lite exportSeed/exportPrivateKeys wipe result.value + the parsed JSON tree.
- Seed-word grid (SplitSeedWords/RenderSeedWordGrid) zeroes its per-frame word
  and cell copies; the wizard word/chip vectors are zeroed before clearing.
- ChatStore::clear() + ResetChatTab() zero decrypted message bodies on
  lock/switch/teardown.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/rpc/rpc_client.cpp

@@ -370,6 +370,20 @@ std::string RPCClient::performCall(const std::string& method, const json& params
 
     json payload = makePayload(method, params);
     std::string body = payload.dump();
+    // Request-side secret scrub (responses are already scrubbed): the body + payload tree can carry
+    // walletpassphrase / encryptwallet / *_importkey args. Wipe them when this scope ends (all return
+    // and throw paths) and detach CURLOPT_POSTFIELDS so the reused handle holds no dangling pointer.
+    struct RequestScrubGuard {
+        CURL* curl; std::string& body; json& payload;
+        ~RequestScrubGuard() {
+            if (!body.empty()) sodium_memzero(&body[0], body.size());
+            scrubJsonSecrets(payload);
+            if (curl) {
+                curl_easy_setopt(curl, CURLOPT_POSTFIELDS, nullptr);
+                curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, 0L);
+            }
+        }
+    } requestScrubGuard{impl_->curl, body, payload};
     std::string response_data;
 
     // Set POST data