refactor: improve code formatting and readability in group duty pin command and tests

- Enhanced the `pin_duty_cmd` function by improving code formatting for better readability, ensuring consistent style across the codebase.
- Updated unit tests for `pin_duty_cmd` to follow the same formatting improvements, enhancing clarity and maintainability.
- No functional changes were made; the focus was solely on code style and organization.

webapp/js/hints.js

@@ -126,9 +126,14 @@ function buildDutyItemTimePrefix(item, idx, total, hintDay, sep, fromLabel, toLa
       timePrefix = toLabel + sep + endHHMM;
     }
   } else if (idx > 0) {
-    if (startHHMM) timePrefix = fromLabel + sep + startHHMM;
-    if (endHHMM && endSameDay && endHHMM !== startHHMM) {
-      timePrefix += (timePrefix ? " " : "") + toLabel + sep + endHHMM;
+    if (startSameDay && startHHMM) {
+      timePrefix = fromLabel + sep + startHHMM;
+      if (endHHMM && endSameDay && endHHMM !== startHHMM) {
+        timePrefix += " " + toLabel + sep + endHHMM;
+      }
+    } else if (endHHMM) {
+      /* Continuation from previous day — only end time */
+      timePrefix = toLabel + sep + endHHMM;
     }
   }
   return timePrefix;