[Feature]: replace default account avatar with witch artwork

[huyanxius]

Problem

The account menu and account profile currently use the Windup bird mark as the default user avatar. This repeats the brand mark in a place where the product mascot should identify the user.

Goal

Replace the default user avatar in both existing account surfaces with the approved low-pixel Windup witch artwork.

Acceptance Criteria

• The top account menu renders the approved witch avatar instead of /windup-mark.svg.
• The account profile header renders the same approved witch avatar.
• The Windup brand mark remains unchanged in brand and navigation locations.
• No avatar picker, upload flow, persistence, or backend contract change is included in this issue.