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/rpc/rpc_client.cpp

@@ -391,6 +391,35 @@ json RPCClient::callSecret(const std::string& method, const json& params)
     }
 }
 
+std::string RPCClient::callSecretString(const std::string& method, const json& params)
+{
+    std::lock_guard<std::recursive_mutex> lk(curl_mutex_);
+    if (!impl_->curl) {
+        throw std::runtime_error("Not connected");
+    }
+
+    long http_code = 0;
+    std::string response_data = performCall(method, params, http_code);
+    try {
+        json result = parseRpcResult(http_code, response_data);
+        std::string out;
+        if (result.is_string()) {
+            // Copy the secret out, then zero the json node's OWN heap buffer — parseRpcResult's
+            // json::parse allocates this independently of response_data, so scrubbing the raw body
+            // alone would leave it behind (B7). `out` is now the only live copy; the caller wipes it.
+            auto& s = result.get_ref<std::string&>();
+            out = s;
+            if (!s.empty()) sodium_memzero(&s[0], s.size());
+        }
+        if (!response_data.empty()) sodium_memzero(&response_data[0], response_data.size());
+        if (!result.is_string()) throw std::runtime_error("RPC result is not a string");
+        return out;
+    } catch (...) {
+        if (!response_data.empty()) sodium_memzero(&response_data[0], response_data.size());
+        throw;
+    }
+}
+
 json RPCClient::call(const std::string& method, const json& params, long timeoutSec)
 {
     std::lock_guard<std::recursive_mutex> lk(curl_mutex_);