RM-337: Show default avatar for gallery submitters without one

[biast12]

RM-337

Description

Gallery submitters without a custom Discord avatar had no image rendered, so on the admin gallery card "by" ran into the username (e.g. "bybiast12"). The backend omits avatar_url for these users.

Every place that shows the submitter now always renders an avatar via userAvatarUrl, which falls back to Discord's default avatar for the user's ID (same approach as BlacklistAvatar in #153):

• Admin gallery card
• Gallery preview modal ("Submitted by")
• Public gallery card
• Public listing page (replaces the generic user icon)

Type of Change

• Bug fix
• New feature
• Breaking change

Testing

• /admin/gallery: a submission from a user with no custom avatar shows "by [default avatar] username · date"; Preview modal shows the same avatar
• /gallery card and /gallery/:id show the default avatar; users with a custom avatar still show theirs

Checklist

• My code follows the style of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works