fix(chat,rpc): address adversarial review of the chat backlog

Four confirmed findings from the review pass:

SECURITY — B7 was incomplete:
- The single-key Export dialog (key_export_dialog) still used plain call() for
  z_exportkey/dumpprivkey/z_exportviewingkey — a live spending-key leak on the
  most common per-address export path, missed by the B7 commit.
- callSecret() zeros the raw body but the parsed json holds its OWN heap copy of
  the secret; several callers did .get<string>() on a temporary json and freed
  that copy un-wiped.
  Fix: add RPCClient::callSecretString() — returns the bare-string result with
  BOTH the raw body AND the json node zeroed, so callers can't forget. Route
  key_export_dialog (×2), exportPrivateKey, and export_all_keys (×2) through it;
  scrub the z_exportmnemonic json node in seed_wallet_creator (object result);
  also wipe the transient key copies, the displayed s_key on reset, and the
  aggregated export-all `keys` buffer.

CHAT:
- Jump-to-latest pill: SetCursorScreenPos moved the parent cursor and never
  restored it, so the composer footer rendered ~8px too high while scrolled up.
  Save + restore the cursor around the pill.
- New-message toast: gating on a chatUnreadCount() watermark delta could be
  swallowed when an outgoing echo (wall-clock) pushed the seen-watermark past a
  later reply's block time. ingest() now reports the cids it appended; the toast
  fires when any is a non-muted conversation — skew-proof, still mute-aware.

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

src/ui/windows/key_export_dialog.cpp

@@ -5,6 +5,7 @@
 #include "key_export_dialog.h"
 #include "../../app.h"
 #include "../../wallet/lite_wallet_controller.h"
+#include <sodium.h>   // sodium_memzero — wipe transient secret copies (B7)
 #include <nlohmann/json.hpp>
 #include "../../rpc/rpc_client.h"
 #include "../../rpc/rpc_worker.h"
@@ -49,7 +50,7 @@ void KeyExportDialog::show(const std::string& address, KeyType type)
     releaseQr();
     s_key_type = type;
     s_address = address;
-    s_key.clear();
+    wallet::secureWipeLiteSecret(s_key);   // zero any prior secret before reuse
     s_error.clear();
 }
 
@@ -62,7 +63,7 @@ void KeyExportDialog::hide()
 {
     s_open = false;
     s_fetching = false;
-    s_key.clear();
+    wallet::secureWipeLiteSecret(s_key);   // zero the displayed secret, don't just drop the buffer
     s_show_key = false;
     s_error.clear();
     releaseQr();
@@ -187,18 +188,18 @@ void KeyExportDialog::render(App* app)
                             std::string error;
                             try {
                                 rpc::RPCClient::TraceScope trace("Settings / Export key");
-                                auto result = rpc->call(method, {addr});
-                                key = result.get<std::string>();
+                                key = rpc->callSecretString(method, {addr});   // scrubs raw body + json node (B7)
                             } catch (const std::exception& e) {
                                 error = e.what();
                             }
-                            return [key, error]() {
+                            return [key = std::move(key), error]() mutable {
                                 if (error.empty()) {
                                     s_key = key;
                                     s_show_key = false;  // Don't show by default
                                 } else {
                                     s_error = error;
                                 }
+                                if (!key.empty()) sodium_memzero(&key[0], key.size());  // wipe the last transient copy
                                 s_fetching = false;
                             };
                         });
@@ -213,18 +214,18 @@ void KeyExportDialog::render(App* app)
                                 std::string error;
                                 try {
                                     rpc::RPCClient::TraceScope trace("Settings / Export viewing key");
-                                    auto result = rpc->call("z_exportviewingkey", {addr});
-                                    key = result.get<std::string>();
+                                    key = rpc->callSecretString("z_exportviewingkey", {addr});  // scrubs raw body + json node (B7)
                                 } catch (const std::exception& e) {
                                     error = e.what();
                                 }
-                                return [key, error]() {
+                                return [key = std::move(key), error]() mutable {
                                     if (error.empty()) {
                                         s_key = key;
                                         s_show_key = true;  // Viewing keys are less sensitive
                                     } else {
                                         s_error = error;
                                     }
+                                    if (!key.empty()) sodium_memzero(&key[0], key.size());  // wipe the last transient copy
                                     s_fetching = false;
                                 };
                             });