fix(client): scope credential cache by coordinator

[YuMo12268]

Summary

• Scope cached credentials by normalized Coordinator address and username.
• Keep credentials for the same username on different Coordinators from overwriting each other.
• Preserve malformed or previous-format credential entries when updating the cache.
• Document that previous-format credential entries are ignored after upgrade.

Validation

• cargo fmt --all
• cargo clippy --workspace
• cargo clippy --workspace --all-features -- -D warnings
• cargo build
• cargo test --workspace
• cargo doc
• cd guide && mdbook build
• Manually verified that:
  • previous-format username:token entries are ignored without panic;
  • credentials for a different Coordinator address are not used for the current Coordinator;
  • logging in writes a new credential entry with coordinator_addr, username, and token;
  • credentials for the same username on different Coordinator addresses can coexist;
  • subsequent auth uses the credential scoped to the current Coordinator.

Notes

• cargo audit reports existing dependency advisories unrelated to this change.

[un-lock-able]

This information doesn't have to be included in the guide. The user doesn't need to know how we store the credentials.