feat(util): app-wide 24h/12h clock format

Add util::formatClockDateTime/formatClockTime driven by a process-wide flag (setClock12h,
synced from the time_format setting each frame). Switch the primary user-facing timestamp
displays to it — the transaction list, wallet-state tx + banned-peer times, the explorer
block time, and the block-info dialog — so one preference drives every clock.

Log files, export filenames/content, console line prefixes, the market chart axis, and the
worker-thread "last updated" string intentionally stay fixed 24h.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/util/text_format.h

@@ -14,6 +14,19 @@
 namespace dragonx {
 namespace util {
 
+// App-wide clock format (24-hour vs 12-hour). Set once per frame by the App from the global
+// `time_format` setting (setClock12h); the formatters below read it so a single preference drives
+// every user-facing timestamp. The Chat tab resolves its own override separately.
+void setClock12h(bool enabled);
+bool clock12h();
+
+// Format an epoch timestamp as a local wall-clock date+time honoring the app clock setting:
+//   24h -> "YYYY-MM-DD HH:MM[:SS]",  12h -> "YYYY-MM-DD hh:MM[:SS] AM/PM". Empty when ts <= 0.
+std::string formatClockDateTime(std::int64_t timestamp, bool withSeconds = false);
+
+// Just the time-of-day portion honoring the clock setting: 24h "HH:MM[:SS]" / 12h "hh:MM[:SS] AM/PM".
+std::string formatClockTime(std::int64_t timestamp, bool withSeconds = false);
+
 // Localized relative time, e.g. "5 minutes ago" (via i18n). Empty when timestamp <= 0.
 std::string formatTimeAgo(std::int64_t timestamp);