perf(chat): use reentrant local time for chat timestamps

The open chat thread called std::localtime at least twice per message
per frame (two day keys for grouping), and more for day separators and
time labels. On glibc every call re-checks the timezone with a stat of
/etc/localtime. The chat time helpers now use util::localTime() from
the new util/local_time.h: localtime_r (localtime_s on Windows), with
no per-call timezone check and no shared static result buffer.

glibc's localtime_r reads the timezone only once, so local_time.h also
adds util::refreshTimezone() (tzset) for caches to call when they
rebuild. Nothing calls it yet; its first caller is the chat thread's
grouping-flags cache in the next commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/util/local_time.h

@@ -0,0 +1,22 @@
+// DragonX Wallet - reentrant local-time helper (no per-call stat of /etc/localtime on glibc).
+#pragma once
+#include <ctime>
+namespace dragonx::util {
+// Fills `out` with the local broken-down time of `t`. Returns false on failure. Thread-safe.
+inline bool localTime(std::time_t t, std::tm& out) {
+#ifdef _WIN32
+    return localtime_s(&out, &t) == 0;
+#else
+    return localtime_r(&t, &out) != nullptr;
+#endif
+}
+// Re-read the TZ database / TZ env (glibc localtime_r does not do this on its own after the first call).
+// Call once per cache rebuild, never per item.
+inline void refreshTimezone() {
+#ifdef _WIN32
+    _tzset();
+#else
+    tzset();
+#endif
+}
+} // namespace dragonx::util