Support browser based oauth2 login and also tackle mcp oauth2 at the same time

[arunanshub]

• ship a cli based simple onboarding logic. safedep auth login. it'll take input from cli only.
• when extending it to support browser login in future, we can expose a separate flag. safedep auth login --web. it'll open / expose a URL to <http://app.safedep.io|app.safedep.io>.
• in future we can slowly make --web the default option and also add a --cli or --tui option to let users use the cli based onboarding.

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