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/ui/windows/chat_tab.cpp

@@ -10,6 +10,7 @@
 #include "../../data/address_book.h"
 #include "../../chat/chat_service.h"
 #include "../../util/i18n.h"
+#include "../../util/local_time.h"   // reentrant localtime (chat day keys / time labels)
 #include "../../util/address_validation.h"  // isShieldedAddress — chat requires a z-address recipient
 #include "../../util/platform.h"        // getConfigDir + writeFileAtomically — conversation export (Q11)
 #include "../../config/settings.h"       // per-conversation mute (Q10)
@@ -134,11 +135,10 @@ std::string shorten(const std::string& s, std::size_t head = 12, std::size_t tai
 
 std::string formatTime(std::int64_t ts) {
     if (ts <= 0) return "";
-    std::time_t t = static_cast<std::time_t>(ts);
-    std::tm* tm = std::localtime(&t);  // UI thread only
-    if (!tm) return "";
+    std::tm tmv;
+    if (!util::localTime(static_cast<std::time_t>(ts), tmv)) return "";
     char buf[40];
-    std::strftime(buf, sizeof(buf), s_chat_time_12h ? "%Y-%m-%d %I:%M %p" : "%Y-%m-%d %H:%M", tm);
+    std::strftime(buf, sizeof(buf), s_chat_time_12h ? "%Y-%m-%d %I:%M %p" : "%Y-%m-%d %H:%M", &tmv);
     return buf;
 }
 
@@ -146,21 +146,19 @@ std::string formatTime(std::int64_t ts) {
 // now lives in a per-day separator, so the group header doesn't repeat it). UI thread only.
 std::string chatTimeOnly(std::int64_t ts) {
     if (ts <= 0) return "";
-    std::time_t t = static_cast<std::time_t>(ts);
-    std::tm* tm = std::localtime(&t);
-    if (!tm) return "";
+    std::tm tmv;
+    if (!util::localTime(static_cast<std::time_t>(ts), tmv)) return "";
     char buf[24];
-    std::strftime(buf, sizeof(buf), s_chat_time_12h ? "%I:%M %p" : "%H:%M", tm);
+    std::strftime(buf, sizeof(buf), s_chat_time_12h ? "%I:%M %p" : "%H:%M", &tmv);
     return buf;
 }
 
 // Local calendar-day key (YYYYMMDD) for detecting day boundaries between messages. UI thread only.
 std::int64_t chatDayKey(std::int64_t ts) {
-    std::time_t t = static_cast<std::time_t>(ts);
-    std::tm* tm = std::localtime(&t);
-    if (!tm) return -1;
-    return static_cast<std::int64_t>(tm->tm_year + 1900) * 10000 +
-           static_cast<std::int64_t>(tm->tm_mon + 1) * 100 + tm->tm_mday;
+    std::tm tmv;
+    if (!util::localTime(static_cast<std::time_t>(ts), tmv)) return -1;
+    return static_cast<std::int64_t>(tmv.tm_year + 1900) * 10000 +
+           static_cast<std::int64_t>(tmv.tm_mon + 1) * 100 + tmv.tm_mday;
 }
 
 // Date-separator label: "Today" / "Yesterday" / "Jul 16, 2026". UI thread only.
@@ -168,11 +166,10 @@ std::string chatDaySeparator(std::int64_t ts, std::int64_t todayKey, std::int64_
     const std::int64_t k = chatDayKey(ts);
     if (k == todayKey) return TR("chat_today");
     if (k == yestKey)  return TR("chat_yesterday");
-    std::time_t t = static_cast<std::time_t>(ts);
-    std::tm* tm = std::localtime(&t);
-    if (!tm) return "";
+    std::tm tmv;
+    if (!util::localTime(static_cast<std::time_t>(ts), tmv)) return "";
     char buf[32];
-    std::strftime(buf, sizeof(buf), "%b %d, %Y", tm);
+    std::strftime(buf, sizeof(buf), "%b %d, %Y", &tmv);
     return buf;
 }
 
@@ -185,11 +182,10 @@ std::string relativeTime(std::int64_t ts) {
     if (d < 3600)        return std::to_string(d / 60)    + "m";
     if (d < 86400)       return std::to_string(d / 3600)  + "h";
     if (d < 7 * 86400)   return std::to_string(d / 86400) + "d";
-    std::time_t t = static_cast<std::time_t>(ts);
-    std::tm* tm = std::localtime(&t);
-    if (!tm) return "";
+    std::tm tmv;
+    if (!util::localTime(static_cast<std::time_t>(ts), tmv)) return "";
     char buf[16];
-    std::strftime(buf, sizeof(buf), "%b %d", tm);
+    std::strftime(buf, sizeof(buf), "%b %d", &tmv);
     return buf;
 }