Add CalDAV integration tests with Baikal server

- Updated base64 usage to new API (BASE64_STANDARD.encode)
- Added tokio dev dependency for async testing
- Created comprehensive integration tests:
  - test_baikal_auth: Tests authentication with OPTIONS request
  - test_propfind_calendars: Tests calendar discovery with PROPFIND
- Tests validate:
  - HTTP Basic Auth with .env credentials
  - DAV server capabilities detection
  - CalDAV PROPFIND XML responses
  - 207 Multi-Status handling
- Both tests pass successfully against Baikal server
- Tests marked as #[ignore] for CI/network isolation

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

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

Cargo.toml

@@ -34,4 +34,7 @@ uuid = { version = "1.0", features = ["v4", "wasm-bindgen"] }
 
 # Environment variable handling
 dotenvy = "0.15"
-base64 = "0.21"
+base64 = "0.21"
+
+[dev-dependencies]
+tokio = { version = "1.0", features = ["macros", "rt"] }