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>

README.md

@@ -16,6 +16,7 @@ A full-stack calendar application built with Rust, featuring a Yew frontend and
 
 - **Frontend**: Yew (Rust WebAssembly)
 - **Backend**: Axum (Rust async web framework)
+- **Shared Models**: RFC 5545-compliant VEvent structures
 - **Protocol**: CalDAV for calendar synchronization
 - **Database**: SQLite (via migrations)
 - **Build Tool**: Trunk for frontend bundling
@@ -37,6 +38,7 @@ A full-stack calendar application built with Rust, featuring a Yew frontend and
 
 2. Start the frontend development server:
    ```bash
+   cd frontend
    trunk serve
    ```
 
@@ -45,12 +47,19 @@ A full-stack calendar application built with Rust, featuring a Yew frontend and
 ### Building for Production
 
 ```bash
+cd frontend
 trunk build --release
 ```
 
 ## Project Structure
 
-- `src/` - Frontend Yew application
+This is a Cargo workspace with the following structure:
+
+- `frontend/` - Yew frontend application
+  - `src/` - Frontend source code
+  - `index.html` - HTML template
+  - `styles.css` - CSS styles
+  - `Trunk.toml` - Trunk build configuration
 - `backend/` - Axum backend server
-- `migrations/` - Database schema migrations
-- `dist/` - Built frontend assets
+- `calendar-models/` - Shared RFC 5545 VEvent models
+- `migrations/` - Database schema migrations