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/daemon/embedded_daemon.h

@@ -79,7 +79,9 @@ public:
     /**
      * @brief Get last error message
      */
-    const std::string& getLastError() const { return last_error_; }
+    // Copy under lock: last_error_ is written from the monitor thread (setState on an unexpected exit)
+    // while the UI thread reads it — a reference would be a torn-read / use-after-free race (M-04).
+    std::string getLastError() const { std::lock_guard<std::mutex> lk(error_mutex_); return last_error_; }
     
     /**
      * @brief Get dragonxd process output (thread-safe copy)
@@ -286,6 +288,7 @@ private:
     std::atomic<State> state_{State::Stopped};
     std::atomic<bool> external_daemon_detected_{false};
     std::string last_error_;
+    mutable std::mutex error_mutex_;    // protects last_error_ (written by main + monitor threads)
     mutable std::mutex output_mutex_;  // protects process_output_
     std::string process_output_;
     StateCallback state_callback_;