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>

CMakeLists.txt

@@ -213,7 +213,7 @@ include(FetchContent)
 FetchContent_Declare(
     json
     GIT_REPOSITORY https://github.com/nlohmann/json.git
-    GIT_TAG v3.11.3
+    GIT_TAG 9cca280a4d0ccf0c08f47a99aa71d1b0e52f8d03  # v3.11.3 — pinned to immutable commit (L-08); tags are mutable
     GIT_SHALLOW TRUE
 )
 FetchContent_MakeAvailable(json)
@@ -222,7 +222,7 @@ FetchContent_MakeAvailable(json)
 FetchContent_Declare(
     tomlplusplus
     GIT_REPOSITORY https://github.com/marzer/tomlplusplus.git
-    GIT_TAG v3.4.0
+    GIT_TAG 30172438cee64926dc41fdd9c11fb3ba5b2ba9de  # v3.4.0 — pinned to immutable commit (L-08); tags are mutable
     GIT_SHALLOW TRUE
 )
 FetchContent_MakeAvailable(tomlplusplus)
@@ -289,7 +289,7 @@ message(STATUS "Fetching libwebp (decode-only, static)...")
 FetchContent_Declare(
     libwebp
     GIT_REPOSITORY https://github.com/webmproject/libwebp.git
-    GIT_TAG v1.4.0
+    GIT_TAG 845d5476a866141ba35ac133f856fa62f0b7445f  # v1.4.0 — pinned to immutable commit (L-08); tags are mutable
     GIT_SHALLOW TRUE
     # libwebp's cpu.cmake applies -mno-sse2/-mno-sse4.1 to its scalar reference DSP
     # files when it can't probe SSE support. Under a macOS universal build