🚸 ♻️ 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/components/substitution-list.vue

@@ -1,5 +1,5 @@
 <script setup>
-import { substitutionsForDate } from "../store";
+import { substitutions } from "../store";
 import { getSubstitutionText, getSubstitutionColor } from "../util";
 import { computed } from "vue";
 
@@ -9,13 +9,13 @@ const props = defineProps({
   },
 });
 
-const substitutions = computed(() => {
-  return substitutionsForDate.value[props.date.setUTCHours(0, 0, 0, 0)];
+const substitutionsForDate = computed(() => {
+  return substitutions.value[props.date.getTime()];
 });
 </script>
 
 <template>
-  <template v-for="substitution in substitutions" :key="substitution">
+  <template v-for="substitution in substitutionsForDate" :key="substitution">
     <div class="substitution" :style="getSubstitutionColor(substitution)">
       <span class="hour">{{ substitution.lesson }}</span>
       <div class="infos">