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>

backend/Cargo.toml

@@ -25,5 +25,13 @@ chrono = { version = "0.4", features = ["serde"] }
 uuid = { version = "1.0", features = ["v4", "serde"] }
 anyhow = "1.0"
 
+# CalDAV dependencies
+reqwest = { version = "0.11", features = ["json"] }
+ical = "0.7"
+regex = "1.0"
+dotenvy = "0.15"
+base64 = "0.21"
+thiserror = "1.0"
+
 [dev-dependencies]
 tokio = { version = "1.0", features = ["macros", "rt"] }