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>

.env.example

@@ -0,0 +1,13 @@
+# CalDAV Server Configuration
+CALDAV_SERVER_URL=https://your-caldav-server.com/dav/
+CALDAV_USERNAME=your-username
+CALDAV_PASSWORD=your-password
+
+# Optional: Calendar collection path (if different from default)
+CALDAV_CALENDAR_PATH=/calendars/your-username/personal/
+
+# Optional: Task/Todo collection path
+CALDAV_TASKS_PATH=/calendars/your-username/tasks/
+
+# Development settings
+RUST_LOG=info