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/ui/windows/block_info_dialog.cpp

@@ -203,10 +203,7 @@ void BlockInfoDialog::render(App* app)
             ImGui::Text("%s", TR("block_timestamp"));
             ImGui::SameLine(lbl.position);
             if (s_block_time > 0) {
-                std::time_t t = static_cast<std::time_t>(s_block_time);
-                char time_buf[64];
-                std::strftime(time_buf, sizeof(time_buf), "%Y-%m-%d %H:%M:%S", std::localtime(&t));
-                ImGui::Text("%s", time_buf);
+                ImGui::Text("%s", dragonx::util::formatClockDateTime(s_block_time, /*withSeconds=*/true).c_str());
             } else {
                 ImGui::TextDisabled("%s", TR("unknown"));
             }