Implement calendar deletion with right-click context menu

Added complete calendar deletion functionality including:
- Context menu component with right-click activation on calendar items
- Backend API endpoint for calendar deletion with CalDAV DELETE method
- Frontend integration with calendar list refresh after deletion
- Fixed URL construction to prevent double /dav.php path issue
- Added proper error handling and user feedback

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

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

styles.css

@@ -1107,4 +1107,66 @@ body {
         width: 100%;
         text-align: center;
     }
+}
+
+/* Context Menu */
+.context-menu {
+    background: white;
+    border: 1px solid #e9ecef;
+    border-radius: 8px;
+    box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+    min-width: 160px;
+    overflow: hidden;
+    animation: contextMenuSlideIn 0.15s ease;
+}
+
+@keyframes contextMenuSlideIn {
+    from {
+        opacity: 0;
+        transform: scale(0.95) translateY(-5px);
+    }
+    to {
+        opacity: 1;
+        transform: scale(1) translateY(0);
+    }
+}
+
+.context-menu-item {
+    display: flex;
+    align-items: center;
+    padding: 0.75rem 1rem;
+    color: #495057;
+    cursor: pointer;
+    transition: background-color 0.2s ease;
+    font-size: 0.9rem;
+    border: none;
+    background: none;
+    width: 100%;
+    text-align: left;
+}
+
+.context-menu-item:hover {
+    background-color: #f8f9fa;
+}
+
+.context-menu-delete {
+    color: #dc3545;
+}
+
+.context-menu-delete:hover {
+    background-color: #f8f9fa;
+    color: #dc3545;
+}
+
+.context-menu-icon {
+    margin-right: 0.5rem;
+    font-size: 1rem;
+}
+
+/* Prevent text selection on context menu items */
+.context-menu-item {
+    user-select: none;
+    -webkit-user-select: none;
+    -moz-user-select: none;
+    -ms-user-select: none;
 }