fix(i18n): thread-safe translation for the daemon monitor thread (don't race loadLanguage)

Concurrency audit: I18n::translate() returns a const char* pointing INTO strings_, and loadLanguage()
clears/rebuilds strings_ on the main thread at a language switch. The EmbeddedDaemon state/monitor callback
builds its status strings via TR() off the main thread, so a concurrent language switch could free the
map node out from under that pointer — a use-after-free data race.

- Add a shared_mutex guarding strings_: loadLanguage() takes it exclusively for the whole clear+rebuild
  (its helpers loadBuiltinEnglish/overlayTranslations are called only from there and don't lock, so no
  recursion); a new translateCopy()/trCopy() takes a shared lock and returns an owned std::string.
- The lock-free translate()/TR() stays for main-thread UI (same thread as the writer, so never races).
- The four daemon-state setDaemonStatus(TR(...)) calls in the monitor callback now use util::trCopy(...).

Synchronization only; no fund-path call touched. Threading fix — clean build + ctest; the race is runtime-only.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -114,6 +114,11 @@ void I18n::overlayTranslations(const nlohmann::json& translations)
 
 bool I18n::loadLanguage(const std::string& locale)
 {
+    // Exclusive for the whole load: strings_ is cleared/rebuilt below, and translateCopy() readers on
+    // other threads (daemon monitor) must never observe it mid-rewrite. loadBuiltinEnglish() and
+    // overlayTranslations() are called only from here and do not lock (no recursion).
+    std::unique_lock<std::shared_mutex> lock(strings_mutex_);
+
     // Try to load from file: CWD-relative first, then exe-relative
     std::string rel_path = "res/lang/" + locale + ".json";
     const std::string exe_dir = Platform::getExecutableDirectory();
@@ -3116,6 +3121,15 @@ const char* I18n::translate(const char* key) const
     return key;
 }
 
+std::string I18n::translateCopy(const char* key) const
+{
+    if (!key) return std::string();
+    std::shared_lock<std::shared_mutex> lock(strings_mutex_);
+    auto it = strings_.find(key);
+    if (it != strings_.end()) return it->second;
+    return std::string(key);
+}
+
 void I18n::registerLanguage(const std::string& code, const std::string& name)
 {
     // Check if already registered