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>

Cargo.toml

@@ -1,66 +1,14 @@
-[package]
-name = "calendar-app"
-version = "0.1.0"
-edition = "2021"
+[workspace]
+members = [
+    "frontend",
+    "backend", 
+    "calendar-models"
+]
+resolver = "2"
 
-# Frontend binary only
-
-[dependencies]
-yew = { version = "0.21", features = ["csr"] }
-web-sys = { version = "0.3", features = [
-    "console",
-    "HtmlSelectElement", 
-    "HtmlInputElement",
-    "HtmlTextAreaElement",
-    "Event",
-    "MouseEvent",
-    "InputEvent",
-    "Element",
-    "Document",
-    "Window",
-    "Location",
-    "Headers",
-    "Request",
-    "RequestInit",
-    "RequestMode",
-    "Response",
-    "CssStyleDeclaration",
-] }
-wasm-bindgen = "0.2"
-
-# HTTP client for CalDAV requests
-reqwest = { version = "0.11", features = ["json"] }
-# Calendar and iCal parsing
-ical = "0.7"
+[workspace.dependencies]
+calendar-models = { path = "calendar-models" }
+chrono = { version = "0.4", features = ["serde"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
-
-# Date and time handling
-chrono = { version = "0.4", features = ["serde", "wasm-bindgen"] }
-chrono-tz = "0.8"
-
-# Error handling
-anyhow = "1.0"
-thiserror = "1.0"
-
-# Logging
-log = "0.4"
-console_log = "1.0"
-
-# UUID generation for calendar events
-uuid = { version = "1.0", features = ["v4", "wasm-bindgen", "serde"] }
-getrandom = { version = "0.2", features = ["js"] }
-
-# Environment variable handling
-dotenvy = "0.15"
-base64 = "0.21"
-
-# XML/Regex parsing
-regex = "1.0"
-
-# Yew routing and local storage (WASM only)
-yew-router = "0.18"
-gloo-storage = "0.3"
-gloo-timers = "0.3"
-wasm-bindgen-futures = "0.4"
-
+uuid = { version = "1.0", features = ["v4", "serde"] }