Add CalDAV dependencies and secure configuration

- Added comprehensive CalDAV/calendar dependencies:
  - reqwest for HTTP requests
  - ical for calendar parsing
  - chrono for date/time handling
  - serde for serialization
  - anyhow/thiserror for error handling
  - uuid for event generation
- Implemented secure config management:
  - dotenvy for environment variable loading
  - base64 for Basic Auth encoding
  - .env.example template for development
  - .gitignore updated to exclude secret files
- Created config.rs module with extensive documentation:
  - CalDAVConfig struct for server credentials
  - Environment-based configuration loading
  - HTTP Basic Auth helper methods
  - Comprehensive error handling
  - Full rustdoc documentation with examples

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

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

.gitignore

@@ -11,4 +11,9 @@ dist/
 .idea/
 
 # Logs
-*.log
+*.log
+
+# Environment variables (secrets)
+.env
+.env.local
+.env.*.local