feat(CalendarView): merge recurring training slots and enhance event filtering

- Implemented a new method to merge recurring training slots with identical weekdays and time windows, improving calendar event management.
- Updated event filtering logic to exclude cancelled training sessions, ensuring only relevant training events are displayed.
- Enhanced the loading process to handle source errors more effectively, improving user experience in the CalendarView.

frontend/src/views/CalendarView.vue

@@ -269,9 +269,10 @@ export default {
           .filter(event => event.type === 'trainingCancellation')
           .flatMap(event => this.getDateKeysForRange(event.date, event.endDate || event.date))
       );
-      this.events = loadedEvents.filter(event => (
+      const afterCancellations = loadedEvents.filter(event => (
         !event.isRecurringTraining || !cancellationDates.has(this.toDateKey(event.date))
       ));
+      this.events = this.mergeRecurringTrainingSlots(afterCancellations);
       this.sourceErrors = sources
         .filter(result => result.status === 'rejected')
         .map(result => result.reason?.source)
@@ -283,6 +284,48 @@ export default {
 
       this.loading = false;
     },
+    /**
+     * Mehrere regelmäßige Trainingszeiten mit identischem Wochentag und gleichem Uhrzeit-Fenster
+     * (z. B. parallel genutzte Gruppen / Dubletten) zu einem Kalendereintrag zusammenführen.
+     */
+    mergeRecurringTrainingSlots(events) {
+      const genericTitle = (t) => !t || /^training$/i.test(String(t).trim());
+      const slotMap = new Map();
+      const passthrough = [];
+      for (const e of events) {
+        if (e.type !== 'training' || !e.isRecurringTraining || !e.time || !e.date) {
+          passthrough.push(e);
+          continue;
+        }
+        const dk = this.toDateKey(e.date);
+        const slotKey = `${dk}|${e.time}`;
+        if (!slotMap.has(slotKey)) {
+          slotMap.set(slotKey, []);
+        }
+        slotMap.get(slotKey).push(e);
+      }
+      const mergedSlots = [];
+      for (const [slotKey, list] of slotMap) {
+        if (list.length === 1) {
+          mergedSlots.push(list[0]);
+          continue;
+        }
+        const sorted = list.slice().sort((a, b) => a.startsAt - b.startsAt);
+        const base = sorted[0];
+        const rawTitles = [...new Set(sorted.map((x) => String(x.title || '').trim()).filter(Boolean))];
+        const specific = rawTitles.filter((t) => !genericTitle(t));
+        const titleJoined = specific.length ? specific.join(' · ') : (rawTitles.join(' · ') || base.title);
+        const safeIdKey = slotKey.replace(/\|/g, '-');
+        mergedSlots.push({
+          ...base,
+          id: `training-merged-${safeIdKey}`,
+          title: titleJoined,
+          subtitle: 'Regelmäßige Trainingszeit',
+          startsAt: Math.min(...sorted.map((x) => x.startsAt))
+        });
+      }
+      return [...passthrough, ...mergedSlots];
+    },
     async loadSource(source, loader) {
       try {
         const events = await loader();