Add external calendars feature: display read-only ICS calendars alongside CalDAV calendars

- Database: Add external_calendars table with user relationships and CRUD operations
- Backend: Implement REST API endpoints for external calendar management and ICS fetching
- Frontend: Add external calendar modal, sidebar section with visibility toggles
- Calendar integration: Merge external events with regular events in unified view
- ICS parsing: Support multiple datetime formats, recurring events, and timezone handling
- Authentication: Integrate with existing JWT token system for user-specific calendars
- UI: Visual distinction with 📅 indicator and separate management section

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

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

frontend/Cargo.toml

@@ -37,6 +37,7 @@ reqwest = { version = "0.11", features = ["json"] }
 ical = "0.7"
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
+serde-wasm-bindgen = "0.6"
 
 # Date and time handling
 chrono = { version = "0.4", features = ["serde", "wasm-bindgen"] }