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>

frontend/Trunk.toml

@@ -0,0 +1,16 @@
+[build]
+target = "index.html"
+dist = "dist"
+
+[env]
+BACKEND_API_URL = "http://localhost:3000/api"
+
+[watch]
+watch = ["src", "Cargo.toml", "../calendar-models/src", "styles.css", "index.html"]
+ignore = ["../backend/", "../target/"]
+
+[serve]
+addresses = ["127.0.0.1"]
+port = 8080
+open = false
+