Add username update to the profile panel

[jenul-ferdinand]

What

Add a username-edit control to the profile panel so a signed-in user can change their username. Wire it to the v2 endpoint PUT /api/v2/users/update/:userId migrated in #267.

Why

• The endpoint exists but nothing calls it.
• The profile panel shows a "More personalisation stuff coming here soon!" placeholder for the current user: profile-panel.component.html. This delivers the first piece of it.

Area

• frontend

Notes or constraints

• The endpoint updates username only. Password update was dropped in Migrate legacy account management endpoints to v2 #267.
• Add a method to user.service.ts and a small form in the panel.
• The server authorises self or admin, so send the current user id.
• Refresh the displayed username after a successful save.

Parent

• Migrate legacy account management endpoints to v2 #267 (migrated the endpoint)
• Tracker: [Parent] Convert the remaining v1 endpoints to v2 #208

Done when

• A signed-in user can rename themselves from the profile panel.
• The new username persists and shows without a manual reload.