Add default-browser check at startup

[lvchenjia]

Description

BrowserProxy can be set as the system default browser by clicking "Set as Default" in the UI. However, there's no feedback if the operation fails, and no way to verify programmatically at launch.

We should add a startup check that detects if BrowserProxy is still the system default browser and warn the user if another app has taken over.

Tasks

• Add a startup check in applicationDidFinishLaunching that checks the current default handler for http/https
• If BrowserProxy is no longer the default, show a notification or update the UI
• This check already exists in Config.isDefaultBrowser - just need to use it
• Write a unit test for the check

Resources

• Config.swift already has isDefaultBrowser computed property
• BrowserProxyApp.swift has the launch flow

Testing

• Launch the app, verify status shows correctly
• Change default browser to Safari, launch again, verify warning appears