Add visibility toggles for CalDAV calendars with event filtering

Users can now toggle visibility of CalDAV calendars using checkboxes in
the sidebar, matching the behavior of external calendars. Events from
hidden calendars are automatically filtered out of the calendar view.

Changes:
- Add is_visible field to CalendarInfo (frontend & backend)
- Add visibility checkboxes to CalDAV calendar list items
- Implement real-time event filtering based on calendar visibility
- Add CSS styling matching external calendar checkboxes
- Default new calendars to visible state

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/styles.css

@@ -3822,6 +3822,29 @@ body {
     flex-shrink: 0;
 }
 
+/* CalDAV Calendar Styles */
+.calendar-info {
+    display: flex;
+    align-items: center;
+    gap: 0.75rem;
+    padding: 0.75rem;
+    border-radius: 8px;
+    transition: all 0.2s ease;
+    cursor: pointer;
+}
+
+.calendar-info:hover {
+    background: rgba(255, 255, 255, 0.1);
+    transform: translateX(2px);
+}
+
+.calendar-info input[type="checkbox"] {
+    width: 16px;
+    height: 16px;
+    accent-color: rgba(255, 255, 255, 0.8);
+    cursor: pointer;
+}
+
 /* Create External Calendar Button */
 .create-external-calendar-button {
     background: rgba(255, 255, 255, 0.15);