Add unit tests for Config and Router

[lvchenjia]

Description

The app currently has no unit tests. We should add basic tests for the core logic: Config loading/saving and Router decision logic.

Tasks

• Create a Tests/ directory with a test target
• Add tests for ConfigData encoding/decoding
• Add tests for Config.browser(for:) logic
• Add tests for Router.runningBrowserIDs() and Router.powerSource()
• Update Package.swift to include test target

Resources

• Use XCTest framework (built-in)
• Tests go in Tests/BrowserProxyTests/

Testing

• Run swift test and verify all tests pass