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/app.cpp

@@ -40,6 +40,7 @@
 #include "ui/windows/faq_dialog.h"
 #include "embedded/IconsMaterialDesign.h"
 #include "ui/windows/key_export_dialog.h"
+#include "ui/imgui_secret_scrub.h"   // scrubImGuiInputTextState — zero ImGui's retained secret buffers
 #include "ui/windows/transaction_details_dialog.h"
 #include "ui/windows/qr_popup_dialog.h"
 #include "ui/windows/validate_address_dialog.h"
@@ -143,6 +144,7 @@ void App::wipeSecrets()
     sodium_memzero(export_result_, sizeof(export_result_));      // exported WIF/z-key (SECRET)
     sodium_memzero(import_key_input_, sizeof(import_key_input_)); // pasted private key (SECRET)
     sodium_memzero(import_clip_preview_, sizeof(import_clip_preview_)); // clipboard preview (SECRET)
+    ui::scrubImGuiInputTextState();   // zero ImGui's retained copies of whatever secret was last typed
 }
 
 // Scrub + close any secret that is currently ON SCREEN (revealed seed phrase, migration seed, exported
@@ -163,6 +165,7 @@ void App::wipeDisplayedSecrets()
     sodium_memzero(export_result_, sizeof(export_result_));
     ui::KeyExportDialog::hide();   // zeros its static s_key via secureWipeLiteSecret
     console_tab_.wipeSecretResult();   // drop any masked key-dump result held in the console
+    ui::scrubImGuiInputTextState();    // zero ImGui's retained copy of the last-typed passphrase/PIN
 }
 
 // Enqueue a blocking xmrig start/stop op onto the dedicated serialized control thread so the render thread
@@ -3486,14 +3489,20 @@ void App::renderLiteFirstRunPrompt()
     ImGui::SetNextWindowPos(ImVec2(vp->Pos.x + vp->Size.x * 0.5f, vp->Pos.y + vp->Size.y * 0.5f),
                             ImGuiCond_Appearing, ImVec2(0.5f, 0.5f));
 
+    // Best-effort zero of the seed-word statics (they hold the whole phrase) before they're dropped.
+    auto wipeStr = [](std::string& s) { if (!s.empty()) sodium_memzero(&s[0], s.size()); s.clear(); };
+    auto wipeWords = [&](std::vector<std::string>& v) { for (auto& w : v) wipeStr(w); v.clear(); };
+    auto wipeChips = [&](std::vector<std::pair<std::string, bool>>& v) {
+        for (auto& c : v) { wipeStr(c.first); c.second = false; } v.clear();
+    };
     auto finish = [&]() {
         wallet::secureWipeLiteSecret(seed);
         sodium_memzero(restoreSeed, sizeof(restoreSeed));
         restoreBirthday = 0;
         restoring = false;
         restoreErr.clear();
-        words.clear();
-        chips.clear();
+        wipeWords(words);
+        wipeChips(chips);
         progress = 0;
         step = 0;
         skipConfirm = false;
@@ -3601,7 +3610,7 @@ void App::renderLiteFirstRunPrompt()
             ImGui::Spacing(); ImGui::Spacing();
 
             if (ui::material::TactileButton(TR("appx_ive_written_it_down"), ImVec2(btnW, 0))) {
-                chips.clear();
+                wipeChips(chips);
                 for (const auto& w : words) chips.emplace_back(w, false);
                 std::mt19937 rng{std::random_device{}()};
                 std::shuffle(chips.begin(), chips.end(), rng);
@@ -4734,7 +4743,7 @@ void App::renderSeedBackupDialog()
         ImGui::Spacing();
         ImGui::TextWrapped("%s", TR("seed_backup_intro"));
         ImGui::Spacing();
-        ui::RenderSeedWordGrid(ui::SplitSeedWords(seed_backup_phrase_));
+        { auto words = ui::SplitSeedWords(seed_backup_phrase_); ui::RenderSeedWordGrid(words); ui::WipeSeedWords(words); }
         ImGui::Spacing();
         if (!seed_backup_status_.empty()) {
             ImGui::PushTextWrapPos(0.0f);
@@ -4904,7 +4913,7 @@ void App::renderSeedMigrationDialog()
         // distinct artifact, not body copy. Purely visual — the words/derivation/logic are untouched.
         {
             ui::material::GlassSectionScope seedPanel;
-            ui::RenderSeedWordGrid(ui::SplitSeedWords(seed_migration_seed_));
+            { auto words = ui::SplitSeedWords(seed_migration_seed_); ui::RenderSeedWordGrid(words); ui::WipeSeedWords(words); }
         }
         ImGui::Spacing();
         ImGui::TextColored(kMedium, "%s", TR("mig_receive_addr"));