feat: let admins edit username on the users screen

[Nithya113]

Summary

• Admins can change a user's username (and existing display name) from the Users edit drawer.
• PATCH /api/users/{id} persists the new username and rekeys the in-memory user cache.
• Later fields in the same PATCH (email, permissions, etc.) use the new username so Save no longer returns 404 after a rename.

Fixes #317

Test plan

• Open Admin → Users on the Vite dashboard (localhost:5173 with local backend).
• Edit a user, change username and display name, Save — expect 200, list shows the new username.
• Confirm PATCH /api/users/{id} in the octop run terminal and no 404.
• Saving with an unchanged username still succeeds.
• Editing only display name still works.

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