Add initial profile setup prompt on first login

[JamnedZ]

The current flow of getting a default display name until changed is mostly fine, but can become a bit messy when people get in the server unattended and forget to change their names. I think it would be nice to show new users a window where they would be able to select a name and, optionally, an avatar before continuing.

[univeous]

I think we should start with a working base version first.
Here's a quick prototype I've implemented:

[univeous]

I'm considering whether to store onboarding state in the database or check it on the frontend.

Frontend check:

• Check if username is the default format (SharkordUser12345), if so, trigger onboarding
• No database changes needed
• Simple and sufficient for the current use case (username + optional avatar and banner)

Database field (hasCompletedOnboarding):

• Requires database migration
• Explicit state tracking
• Could be useful if onboarding expands in the future (beyond just changing display name and avatar)

[ivan-vrabac]

I'm considering whether to store onboarding state in the database or check it on the frontend.

Frontend check:

• Check if username is the default format (SharkordUser12345), if so, trigger onboarding
• No database changes needed
• Simple and sufficient for the current use case (username + optional avatar and banner)

Database field (hasCompletedOnboarding):

• Requires database migration
• Explicit state tracking
• Could be useful if onboarding expands in the future (beyond just changing display name and avatar)

Onboarding sounds nice. How difficult would DB migration be?

[univeous]

How difficult would DB migration be?

I'd say it's not that difficult.
The DB approach looks more reasonable, especially if the OIDC PR (#237) gets merged — OIDC users may bring their own usernames, which would make frontend detection tricky or cumbersome.