[FEATURE]: Add Username edit section to Profile page with locked-once validation

[Gautam25Raj]

Affected Apps / Packages

Studio (apps/studio)

Is your feature request related to a problem?

Yes. The backend database schema and endpoints support setting a unique user username via PUT /me/username, which is a hard prerequisite for creating public share links (under /share/[username]/[slug]).

Currently, there is no UI section or input field in the Profile page allowing users to view or set their username. Furthermore, the backend enforces a constraint where a username can only be set once (returns 409 if modified again). The UI must implement this field with clear warnings and handle the edit lock.

• Branch Name: feat/studio-profile-username-setup
• PR Title: [Feature] [Studio]: add username configuration section to profile

Describe the solution you'd like

• A "Username" section should be displayed in the ProfileDataPanel.tsx.
• If no username is set, it should show a warning/note: "Username can only be set once and cannot be changed later." and an "Edit" button.
• Clicking "Edit" opens a modal EditProfileUsernameModal where the user can check availability and set it.
• If a username is already set, clicking "Edit" should open a modal that states: "Username is locked and cannot be changed. Please contact our support team if you need assistance."

Describe alternatives you've considered

• Leaving the username unset or forcing manual setting through DB scripting, which locks users out of the share feature.
• Allowing changing the username in the UI, which would result in failed requests since the backend rejects modifications.

Additional Context

• Database field: User.username (unique string).
• Backend service logic: UserService.updateUsername(userId, username) raises 409 "Username is locked and cannot be changed" if current.username is already set.