fix: security-audit remediation (15 findings), empty-wallet warning, and send/chat/console/shutdown UX

Security audit remediation (15 confirmed findings from the codebase audit):
- H-02: scrub+delete the decrypt-flow plaintext key export on ALL exit paths
  (RAII guard) and purge stale obsidiandecryptexport* files at startup.
- M-01/L-03/L-04/L-05/L-07: sodium_memzero the Set-PIN and encrypt-PIN worker
  passphrase/PIN copies, the RPC Basic-auth string (auth_), the exported/imported
  key buffers (App::wipeSecrets, called from ~App and before main's _Exit), and
  the first-run wizard "Skip" buffers.
- M-03/M-04/M-05/L-06: return locked COPIES from XmrigManager/EmbeddedDaemon
  getters (dedicated error_mutex_; DaemonController::lastError now by value),
  route xmrig last_error_ writes through a locked setter, and wrap
  shutdown_status_/wizard_stop_status_ in a locking GuardedStatus
  (wizard_stopping_external_ -> std::atomic).
- M-02: persist after a console send/shield/import in the lite backend.
- L-01: require the confirm click for z_shieldcoinbase/z_mergetoaddress.
- L-02: quote/escape each Windows daemon argv per the MSDN CommandLineToArgvW rules.
- L-08: pin json/tomlplusplus/libwebp FetchContent to immutable commit SHAs.
- I-01: extract updater archives from the already-verified in-memory buffer
  (no disk re-read TOCTOU).

Feature: warn once (full-node) when the active wallet loads empty while a sibling
wallet file in the datadir holds keys. A funded salvage wallet.<ts>.bak routes to
the recovery/Restore flow; a funded sibling .dat routes to the wallet manager.
Per-wallet-file dismissal; gated on synced + address-list-loaded to avoid false
positives on warm reconnect / spent-down wallets.

UX fixes:
- send: show the TOTAL balance (with a spendable "available" note) in the source
  dropdown and keep pending-change addresses visible.
- chat: insert emoji at the cursor position; restrict new-chat recipients to
  shielded (z) addresses.
- console: optional auto-focus of the command input on tab open (off by default).
- shutdown: when "stop external daemon" is on, keep the shutdown screen up until
  the external node actually exits, showing live status.

Adversarially reviewed; verified across full-node, lite, and Windows builds; tests pass.

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

src/app_wizard.cpp

@@ -889,8 +889,9 @@ void App::renderFirstRunWizard() {
                 }
 
                 if (wizard_stopping_external_) {
+                    const std::string ws = wizard_stop_status_.get();
                     dl->AddText(captionFont, captionFont->LegacySize, ImVec2(cx, cy), dimCol,
-                                wizard_stop_status_.c_str());
+                                ws.c_str());
                     cy += captionFont->LegacySize + 8.0f * dp;
                 } else {
                     float stopW = 150.0f * dp;
@@ -1376,6 +1377,13 @@ void App::renderFirstRunWizard() {
                         encrypt_status_ = TR("wiz_skip_confirm");
                     } else {
                         s_skipEncConfirm = false;
+                        // Skipping leaves the wallet UNENCRYPTED — wipe the passphrase/PIN the user may have
+                        // typed so it doesn't linger in these process-lifetime buffers (only the Encrypt
+                        // path cleared them before). (L-07)
+                        memset(encrypt_pass_buf_, 0, sizeof(encrypt_pass_buf_));
+                        memset(encrypt_confirm_buf_, 0, sizeof(encrypt_confirm_buf_));
+                        memset(wizard_pin_buf_, 0, sizeof(wizard_pin_buf_));
+                        memset(wizard_pin_confirm_buf_, 0, sizeof(wizard_pin_confirm_buf_));
                         wizard_phase_ = WizardPhase::Done;
                         settings_->setWizardCompleted(true);
                         settings_->save();