feat(sidebar): distinct share icon for groupVisible, hide scrollbars

- groupVisible calendars get a share icon instead of the people icon;
  people icon is reserved for actual group calendars
- sidebar scrollbar hidden (scrollbar-width: none + webkit)
- modal-body scrollbar hidden — fixes Impressum scroll indicator

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/css/app.css

@@ -508,7 +508,9 @@ a { color: var(--primary); text-decoration: none; }
   overflow-y: auto;
   overflow-x: hidden;
   transition: transform var(--transition);
+  scrollbar-width: none;
 }
+.sidebar::-webkit-scrollbar { display: none; }
 .sidebar.collapsed { transform: translateX(calc(-1 * var(--sidebar-w))); margin-right: calc(-1 * var(--sidebar-w)); }
 .sidebar-inner { padding-bottom: 8px; }
 .sidebar-copyright {
@@ -1066,7 +1068,8 @@ a { color: var(--primary); text-decoration: none; }
 }
 .modal-header h3 { font-size: 18px; font-weight: 500; flex: 1; }
 .modal-close { font-size: 24px; }
-.modal-body { padding: 20px; overflow-y: auto; flex: 1; }
+.modal-body { padding: 20px; overflow-y: auto; flex: 1; scrollbar-width: none; }
+.modal-body::-webkit-scrollbar { display: none; }
 .modal-body p { margin: 0 0 14px; font-size: 14px; color: var(--text-1); }
 .modal-body p:last-child { margin-bottom: 0; }
 .modal-body a { color: var(--primary); text-decoration: none; }