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/ui/windows/chat_tab.cpp

@@ -2484,11 +2484,16 @@ void ResetChatTab()
     s_show_blocked = false;
     // Drop the per-frame memoization caches so the next wallet doesn't briefly render the previous one's
     // conversations/thread (store.revision() is monotonic and would rebuild anyway, but be explicit).
+    // Zero the decrypted plaintext the memo caches hold (conversation previews + full thread bodies)
+    // before dropping them, so the previous wallet's messages don't linger in freed heap.
+    auto wipeStr2 = [](std::string& s) { if (!s.empty()) sodium_memzero(&s[0], s.size()); s.clear(); };
+    for (auto& c : s_convs) wipeStr2(c.lastBody);
     s_convs.clear();
     s_convsHidden = 0;
     s_convsKey = ~0ull;
     s_threadCid.clear();
     s_threadRev = ~0ull;
+    for (auto& m : s_threadMsgs) { wipeStr2(m.body); wipeStr2(m.fail_reason); wipeStr2(m.peer_zaddr); }
     s_threadMsgs.clear();
     s_threadFlags.clear(); s_threadRows.clear(); s_threadHeights.clear();
     s_threadPrefix.assign(1, 0.0f); s_threadFailed.clear(); s_threadBodySizes.clear();

src/ui/windows/seed_display.h

@@ -8,6 +8,7 @@
 #include <cstdio>
 #include <string>
 #include <vector>
+#include <sodium.h>
 
 #include "imgui.h"
 #include "../layout.h"   // Layout::dpiScale()
@@ -15,19 +16,31 @@
 namespace dragonx {
 namespace ui {
 
+// Best-effort zero of a seed-word vector (each word holds part of the phrase). Call after rendering.
+inline void WipeSeedWords(std::vector<std::string>& words)
+{
+    for (auto& w : words) { if (!w.empty()) sodium_memzero(&w[0], w.size()); w.clear(); }
+    words.clear();
+}
+
 // Split a whitespace-separated seed phrase into its individual words.
 inline std::vector<std::string> SplitSeedWords(const std::string& phrase)
 {
     std::vector<std::string> words;
+    words.reserve(32);   // avoid reallocation stranding un-wiped moved-from copies in freed heap
     std::string cur;
     for (char c : phrase) {
         if (c == ' ' || c == '\t' || c == '\n' || c == '\r') {
-            if (!cur.empty()) { words.push_back(cur); cur.clear(); }
+            if (!cur.empty()) {
+                words.push_back(cur);
+                sodium_memzero(&cur[0], cur.size());   // zero the scratch copy before reuse
+                cur.clear();
+            }
         } else {
             cur.push_back(c);
         }
     }
-    if (!cur.empty()) words.push_back(cur);
+    if (!cur.empty()) { words.push_back(cur); sodium_memzero(&cur[0], cur.size()); cur.clear(); }
     return words;
 }
 
@@ -45,6 +58,7 @@ inline void RenderSeedWordGrid(const std::vector<std::string>& words, float colS
         std::snprintf(cell, sizeof(cell), "%2zu. %s", i + 1, words[i].c_str());
         const float need = ImGui::CalcTextSize(cell).x + 20.0f * dp;  // cell width + inter-column gap
         if (need > colStride) colStride = need;
+        sodium_memzero(cell, sizeof(cell));   // the cell held a seed word
     }
     for (size_t i = 0; i < words.size(); ++i) {
         char cell[96];
@@ -52,6 +66,7 @@ inline void RenderSeedWordGrid(const std::vector<std::string>& words, float colS
         ImGui::TextUnformatted(cell);
         if ((i % 4) != 3 && i + 1 < words.size())
             ImGui::SameLine(((i % 4) + 1) * colStride);
+        sodium_memzero(cell, sizeof(cell));
     }
 }