Add JSDoc documentation and file headers to all source files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/module/abbreviateNumber.js

@@ -1,7 +1,18 @@
+// The Lucia project.
+// Copyright 2023-2026 DSP, inc.  All rights reserved.
+// Authors:
+//   chiayin.kuo@dsp.im (chiayin), 2023/1/31
+//   imacat.yang@dsp.im (imacat), 2023/9/23
+//   cindy.chang@dsp.im (Cindy Chang), 2024/5/30
+/** @module abbreviateNumber Duration abbreviation formatting. */
+
 /**
- * 將數字轉換成簡寫的形式，設定 dhms 的數值
- * @param {number} totalSeconds 總秒數
- * @returns {string}
+ * Converts a total number of seconds into a human-readable abbreviated
+ * duration string using days, hours, minutes, and seconds.
+ *
+ * @param {number} totalSeconds - The total number of seconds to convert.
+ * @returns {string} The abbreviated duration string (e.g. "2d 3h 15m 30s"),
+ *   or "0" if totalSeconds is zero.
  */
 export default function abbreviateNumber(totalSeconds) {
   let seconds = 0;