Make groupDocsByDay keep date time of day

instead of stringyfied key

aleksis/apps/alsijil/frontend/components/Coursebook.vue

@@ -76,19 +76,20 @@ export default {
     },
   },
   methods: {
+    // => [[dt doc ...] ...]
     groupDocsByDay(docs) {
       const byDay = docs.reduce(
         (byDay, doc) => {
           // This works with dummy. Does actual doc have dateStart instead?
           const day = DateTime.fromISO(doc.datetimeStart).startOf('day');
-          byDay[day] ??= [];
+          byDay[day] ??= [day];
           byDay[day].push(doc);
           return byDay;
         },
         {}
       );
 
-      return Object.keys(byDay).sort().map(key => [key, ...byDay[key]]);
+      return Object.keys(byDay).sort().map(key => byDay[key]);
     },
   },
 };