Add chapter duration next to start time.

This adds the duration to the chapter table on the book page.

components/tables/ChaptersTable.vue

@@ -15,6 +15,7 @@
         <tr>
           <th class="text-left">{{ $strings.LabelTitle }}</th>
           <th class="text-center w-16">{{ $strings.LabelStart }}</th>
+          <th class="text-center w-16">{{ $strings.LabelDuration }}</th>
         </tr>
         <tr v-for="chapter in chapters" :key="chapter.id">
           <td>
@@ -23,6 +24,9 @@
           <td class="font-mono text-center underline w-16" @click.stop="goToTimestamp(chapter.start)">
             {{ $secondsToTimestamp(chapter.start) }}
           </td>
+          <td class="font-mono text-center">
+            {{ $secondsToTimestamp(Math.max(0, chapter.end - chapter.start)) }}
+          </td>
         </tr>
       </table>
     </transition>