Implement shared RFC 5545 VEvent library with workspace restructuring

- Created calendar-models/ shared library with RFC 5545-compliant VEvent structures
- Migrated backend to use shared VEvent with proper field mappings (dtstart/dtend, rrule, exdate, etc.)
- Converted CalDAV client to parse into VEvent structures with structured types
- Updated all CRUD handlers to use VEvent with CalendarUser, Attendee, VAlarm types
- Restructured project as Cargo workspace with frontend/, backend/, calendar-models/
- Updated Trunk configuration for new directory structure
- Fixed all compilation errors and field references throughout codebase
- Updated documentation and build instructions for workspace structure

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

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

calendar-models/src/lib.rs

@@ -0,0 +1,10 @@
+//! RFC 5545 Compliant Calendar Models
+//! 
+//! This crate provides shared data structures for calendar applications
+//! that comply with RFC 5545 (iCalendar) specification.
+
+pub mod vevent;
+pub mod common;
+
+pub use vevent::*;
+pub use common::*;