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/daemon/seed_wallet_creator.cpp

@@ -111,8 +111,12 @@ SeedWalletResult SeedWalletCreator::create(bool keepDatadir,
     // 6. Export the new seed phrase + a fresh shielded receive address (the future sweep target).
     try {
         auto m = cli.callSecret("z_exportmnemonic");   // zero the raw body too (B7)
-        if (m.contains("mnemonic") && m["mnemonic"].is_string())
-            r.seedPhrase = m["mnemonic"].get<std::string>();
+        if (m.contains("mnemonic") && m["mnemonic"].is_string()) {
+            // Take our copy, then scrub the json node's own copy so it isn't freed in the clear (B7).
+            auto& mn = m["mnemonic"].get_ref<std::string&>();
+            r.seedPhrase = mn;
+            if (!mn.empty()) sodium_memzero(&mn[0], mn.size());
+        }
         r.destAddress = cli.call("z_getnewaddress").get<std::string>();
         r.ok = !r.seedPhrase.empty() && !r.destAddress.empty();
         if (!r.ok) r.error = "The isolated node returned an empty seed or address.";