fix: allow duration of playlist entries to be reset to undefined

undefined duration has special handling in the playout so this should be
supported.

src/components/playlist/APlaylistEntryEditor.vue

@@ -92,8 +92,8 @@
             :value="typeof duration === 'number' ? secondsToDurationString(duration) : ''"
             class="md:tw-max-w-[200px]"
             :placeholder="t('file.durationPlaceholder')"
-            @blur="duration = parseTime(($event.target as HTMLInputElement).value)"
-            @keydown.enter="duration = parseTime(($event.target as HTMLInputElement).value)"
+            @blur="setDuration"
+            @keydown.enter="setDuration"
           />
         </FormGroup>
       </FormTable>
@@ -189,6 +189,13 @@ const artistInfo = computed(() => {
   }
 })
 
+function setDuration(event: Event) {
+  const value = (event.target as HTMLInputElement).value
+  // A duration of zero should be treated as undefined so that the
+  // default playout behaviour for non-fixed-lengths entries can take over.
+  duration.value = parseTime(value) || undefined
+}
+
 async function save() {
   if (duration.value !== entry.value.duration) {
     entry.value = { ...entry.value, duration: duration.value }