🚸 ♻️ Refactor store and improve UX

- Move view related functions out of the store
- Restructure the store.js file
- Improve error handling of `/check`
- Show red loading spinner if data fetch failed
- Prefetch current, next and previous day instead of only current
- Refactor some other frontend file

src/util.js

@@ -33,3 +33,11 @@ export function getDateSkippingWeekend(date, onlyNext) {
     );
   return date;
 }
+
+export function getNextAndPrevDay(date) {
+  return [
+    new Date(date.getTime()),
+    getDateSkippingWeekend(new Date(date.getTime() + 86400000)),
+    getDateSkippingWeekend(new Date(date.getTime() - 86400000)),
+  ].map((e) => e.getTime());
+}