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/export_all_keys_dialog.cpp

@@ -4,6 +4,7 @@
 
 #include "export_all_keys_dialog.h"
 #include "../../app.h"
+#include <sodium.h>   // sodium_memzero — wipe exported key material from memory (B7)
 #include "../../rpc/rpc_client.h"
 #include "../../rpc/rpc_worker.h"
 #include "../../util/i18n.h"
@@ -180,13 +181,12 @@ void ExportAllKeysDialog::render(App* app)
                             for (const auto& addr : z_addrs) {
                                 try {
                                     rpc::RPCClient::TraceScope trace("Settings / Export all keys");
-                                    auto result = rpc->callSecret("z_exportkey", {addr});   // zero the raw body too (B7)
-                                    if (result.is_string()) {
-                                        keys += "# Address: " + addr + "\n";
-                                        keys += result.get<std::string>() + "\n\n";
-                                        exported++;   // count only real successes (locked/failed keys don't count)
-                                    }
-                                } catch (...) {}
+                                    std::string k = rpc->callSecretString("z_exportkey", {addr});  // scrubs raw body + json node (B7)
+                                    keys += "# Address: " + addr + "\n";
+                                    keys += k + "\n\n";
+                                    exported++;   // count only real successes (locked/failed keys don't count)
+                                    if (!k.empty()) sodium_memzero(&k[0], k.size());  // wipe the transient copy
+                                } catch (...) {}  // non-string / locked key → skip (not counted)
                             }
                         }
                         
@@ -196,13 +196,12 @@ void ExportAllKeysDialog::render(App* app)
                             for (const auto& addr : t_addrs) {
                                 try {
                                     rpc::RPCClient::TraceScope trace("Settings / Export all keys");
-                                    auto result = rpc->callSecret("dumpprivkey", {addr});   // zero the raw body too (B7)
-                                    if (result.is_string()) {
-                                        keys += "# Address: " + addr + "\n";
-                                        keys += result.get<std::string>() + "\n\n";
-                                        exported++;   // count only real successes (locked/failed keys don't count)
-                                    }
-                                } catch (...) {}
+                                    std::string k = rpc->callSecretString("dumpprivkey", {addr});  // scrubs raw body + json node (B7)
+                                    keys += "# Address: " + addr + "\n";
+                                    keys += k + "\n\n";
+                                    exported++;   // count only real successes (locked/failed keys don't count)
+                                    if (!k.empty()) sodium_memzero(&k[0], k.size());  // wipe the transient copy
+                                } catch (...) {}  // non-string / locked key → skip (not counted)
                             }
                         }
                         
@@ -219,6 +218,7 @@ void ExportAllKeysDialog::render(App* app)
                                 writeOk = true;
                             }
                         }
+                        if (!keys.empty()) sodium_memzero(&keys[0], keys.size());  // don't leave every key in freed heap
 
                         return [exported, total, filepath, writeOk]() {
                             s_exported_count = exported;