feat: flache sortierbare Kalenderliste (Drag&Drop) + Fixes

- Sidebar: eine flache Kalenderliste statt Quellen-Gruppen; Quelle/Konto
  klein-grau inline rechts neben dem Namen; per Drag&Drop sortierbar
  (Reihenfolge pro Geraet in localStorage).
- Gruppenkalender serverseitig auch beim Besitzer als group:true markiert
  -> erscheint nicht mehr in der "Fuer Gruppen sichtbar"-Auswahl und nicht
  in der normalen Kalenderliste (nur unter Gruppen).
- Settings-URL-State: uiSettingsOpen wird beim Init aus der URL gesetzt,
  bevor das erste writeUrlState() es ueberschreibt -> Reload bleibt jetzt
  wirklich in den Einstellungen.
- Auswahl-Markierungen (Mitglieder/Gruppen-Sichtbar) in Akzentfarbe,
  CSS-gezeichnet statt blauer Emoji. Version v28.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/routers/local_router.py

@@ -91,13 +91,34 @@ def list_calendars(
     db: Session = Depends(get_db),
     current_user: models.User = Depends(get_current_user),
 ):
+    # Map calendar_id -> group name for every group the user belongs to, so we
+    # can flag group calendars as such even when the user owns them (the creator
+    # owns the group calendar — it must still be marked group:true).
+    group_cal_map = {
+        cal_id: name
+        for cal_id, name in (
+            db.query(models.GroupCalendar.calendar_id, models.Group.name)
+            .join(models.Group, models.Group.id == models.GroupCalendar.group_id)
+            .join(models.GroupMember, models.GroupMember.group_id == models.GroupCalendar.group_id)
+            .filter(models.GroupMember.user_id == current_user.id)
+            .all()
+        )
+    }
+
     # Own calendars
     own = (
         db.query(models.LocalCalendar)
         .filter(models.LocalCalendar.user_id == current_user.id)
         .all()
     )
-    result = [_cal_dict(c, owned=True) for c in own]
+    result = []
+    for c in own:
+        d = _cal_dict(c, owned=True)
+        if c.id in group_cal_map:
+            d["group"] = True
+        result.append(d)
+
+    seen_ids = {c.id for c in own}
 
     # Calendars shared with this user
     shares = (
@@ -105,33 +126,30 @@ def list_calendars(
         .filter(models.CalendarShare.user_id == current_user.id)
         .all()
     )
-    seen_ids = {c.id for c in own}
     for share in shares:
         cal = share.calendar
         if cal is None or cal.id in seen_ids:
             continue
         seen_ids.add(cal.id)
         owner = db.query(models.User).filter(models.User.id == cal.user_id).first()
-        result.append(_cal_dict(
+        d = _cal_dict(
             cal, owned=False,
             shared_by=owner.username if owner else None,
             permission=share.permission,
-        ))
+        )
+        if cal.id in group_cal_map:
+            d["group"] = True
+        result.append(d)
 
-    # Group calendars the user can reach via membership (read_write), so members
-    # can select the group calendar in the editor and see it in their list.
-    group_cals = (
-        db.query(models.LocalCalendar, models.Group.name)
-        .join(models.GroupCalendar, models.GroupCalendar.calendar_id == models.LocalCalendar.id)
-        .join(models.Group, models.Group.id == models.GroupCalendar.group_id)
-        .join(models.GroupMember, models.GroupMember.group_id == models.GroupCalendar.group_id)
-        .filter(models.GroupMember.user_id == current_user.id)
-        .all()
-    )
-    for cal, group_name in group_cals:
-        if cal.id in seen_ids:
+    # Group calendars reached via membership (read_write) that aren't already
+    # listed, so members can select/see the group calendar.
+    for cal_id, group_name in group_cal_map.items():
+        if cal_id in seen_ids:
             continue
-        seen_ids.add(cal.id)
+        cal = db.query(models.LocalCalendar).filter(models.LocalCalendar.id == cal_id).first()
+        if not cal:
+            continue
+        seen_ids.add(cal_id)
         d = _cal_dict(cal, owned=False, shared_by=group_name, permission="read_write")
         d["group"] = True
         result.append(d)