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>

tests/test_phase4.cpp

@@ -64,6 +64,7 @@
 #include "data/seed_migration_resume.h"
 #include "util/address_validation.h"
 #include "util/seed_phrase.h"
+#include "util/local_time.h"
 #include "util/image_decode_core.h"
 #include "daemon/daemon_startup_diagnosis.h"
 #include "util/amount_format.h"
@@ -14408,6 +14409,26 @@ void testConsolidationRunSimulation()
     EXPECT_TRUE(runVerdict(sum) == RunVerdict::Clean);
 }
 
+namespace {
+void testLocalTime()
+{
+#ifndef _WIN32
+    const char* prev = std::getenv("TZ");
+    const bool hadTz = prev != nullptr;
+    const std::string savedTz = hadTz ? prev : "";
+    setenv("TZ", "XXX-3", 1); dragonx::util::refreshTimezone();   // POSIX TZ: UTC+3, no tzdata needed
+    std::tm t{};
+    EXPECT_TRUE(dragonx::util::localTime(0, t));
+    EXPECT_EQ(t.tm_year, 70); EXPECT_EQ(t.tm_mday, 1); EXPECT_EQ(t.tm_hour, 3);
+    setenv("TZ", "YYY5", 1); dragonx::util::refreshTimezone();    // UTC-5: refreshTimezone() must pick it up
+    EXPECT_TRUE(dragonx::util::localTime(0, t));
+    EXPECT_EQ(t.tm_mday, 31); EXPECT_EQ(t.tm_hour, 19);
+    if (hadTz) setenv("TZ", savedTz.c_str(), 1); else unsetenv("TZ");
+    dragonx::util::refreshTimezone();
+#endif
+}
+}  // namespace
+
 int main()
 {
     testConnectionConfig();
@@ -14644,6 +14665,7 @@ int main()
     testImageDecodeCore();
     testImageDecodeJobs();
     testGeneratedResourceBehavior();
+    testLocalTime();
 
     if (g_failures != 0) {
         std::cerr << g_failures << " test assertion(s) failed\n";