Implement complete CalDAV events integration

Added full CalDAV integration to display real calendar events from Baikal server:

Backend changes:
- Added CalDAV client with iCalendar parsing and XML handling
- Created /api/calendar/events endpoint with authentication
- Fixed multiline regex patterns for namespace-prefixed XML tags
- Added proper calendar path discovery and event filtering

Frontend changes:
- Created CalendarService for API communication
- Updated calendar UI to display events as blue boxes with titles
- Added loading states and error handling
- Integrated real-time event fetching on calendar load

Now successfully displays 3 test events from the Baikal server with proper date formatting and responsive design.

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

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

index.html

@@ -188,6 +188,26 @@
             flex-direction: column;
         }
 
+        .calendar-loading, .calendar-error {
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            height: 100%;
+            background: white;
+            border-radius: 12px;
+            box-shadow: 0 4px 16px rgba(0,0,0,0.1);
+        }
+
+        .calendar-loading p {
+            font-size: 1.2rem;
+            color: #666;
+        }
+
+        .calendar-error p {
+            font-size: 1.2rem;
+            color: #d32f2f;
+        }
+
         /* Calendar Component */
         .calendar {
             background: white;
@@ -308,6 +328,24 @@
             gap: 2px;
         }
 
+        .event-box {
+            background: #2196f3;
+            color: white;
+            padding: 2px 4px;
+            border-radius: 3px;
+            font-size: 0.7rem;
+            line-height: 1.2;
+            overflow: hidden;
+            text-overflow: ellipsis;
+            white-space: nowrap;
+            cursor: pointer;
+            transition: background-color 0.2s;
+        }
+
+        .event-box:hover {
+            background: #1976d2;
+        }
+
         .event-dot {
             background: #ff9800;
             height: 6px;