Add Internet Identity login flow

[humandebri]

Summary

• Add Internet Identity (II) login flow with local callback handling.
• Add identity storage helpers and update CLI options/docs for II usage.
• Improve login responses and error handling.

Changes - New II login command and callback handling code.

• Identity store utilities (atomic write, restricted permissions).
• CLI argument updates and documentation tweaks.
• Python bindings updated for dfx identity use.

Notes

• The II login flow uses a local callback on 127.0.0.1:8620.

[ClankPan]

@humandebri

That’s awesome—thank you for the PR!
I’m looking through your code, but it’s a bit tricky to follow. Could you clarify how the CLI logs in via Internet Identity? It would really help if you could add an architecture overview and some comments so I can understand the flow better.

[humandebri]

Thanks for the review! I’ve added a short architecture overview in ii-login-architecture.mdand expanded inline comments in the II login flow.
I also replaced the custom HTTP server with axum, implemented CSRF-style state validation, added a 5‑minute timeout, and ensured the callback server only accepts a single successful request.
For safety, the callback now enforces Content-Length, JSON Content-Type, and a 256KB body limit. I also renamed the module to make the II-specific login clearer.