Fix external calendar timezone conversion and styling

- Add comprehensive Windows timezone support for global external calendars
  - Map Windows timezone names (e.g. "Mountain Standard Time") to IANA zones (e.g. "America/Denver")
  - Support 60+ timezone mappings across North America, Europe, Asia, Asia Pacific, Africa, South America
  - Add chrono-tz dependency for proper timezone handling
- Fix external calendar event colors by setting calendar_path for color lookup
- Add visual distinction for external calendar events with dashed borders and calendar emoji
- Update timezone parsing to extract TZID parameters from iCalendar DTSTART/DTEND properties
- Pass external calendar data through component hierarchy for color matching

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

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

backend/Cargo.toml

@@ -22,6 +22,7 @@ hyper = { version = "1.0", features = ["full"] }
 serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 chrono = { version = "0.4", features = ["serde"] }
+chrono-tz = "0.8"
 uuid = { version = "1.0", features = ["v4", "serde"] }
 anyhow = "1.0"