feat: add --client to user assign-role/remove-role

[NathaelB]

Summary

--client <client_id> on user assign-role and user remove-role — resolves the named role among that client's roles instead of realm roles, mirroring realm role's existing --client support (#30/#33).

Issue

Closes #23 — this was the one piece left over from #29/#31, previously deferred because I didn't want to guess a doubly-nested assignment endpoint. Turns out none is needed: verified live in #37 that assign_user_role/remove_user_role accept a client role id through the exact same endpoint as a realm role id (role ids are unique across both scopes — same finding as delete_role in #33). So this PR is just wiring --client through to role resolution; the assignment calls themselves are unchanged.

Stacked on #38 → #37 → #36 → #35

Same chain as the last few: branched from feat/client-secret-command (#38). Merge in order: #35, #36, #37, #38, then this one.

Verification

Ran live against a local FerrisKey server: created a client with a client-scoped role and a throwaway user, ran assign-role --client, confirmed via user roles, ran remove-role --client, confirmed it's gone. Cleaned up all fixtures.

Test plan

• cargo build --workspace
• cargo test --workspace (80 passed)
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• Manual assign/remove of a client role against a running FerrisKey server (see above)

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