fix: Remove default background color from AvatarFallback

[robdimarco-atxp]

Problem

When passing custom background colors to AvatarFallback via className (e.g., bg-indigo-500 text-white), the default bg-muted class was creating conflicts. Even though cn() uses tailwind-merge, having default background colors on a component designed to accept custom colors leads to:

• Confusion about what color will actually render
• Potential CSS specificity issues
• Inconsistent behavior across contexts

This was discovered when implementing consistent avatar colors in LibreChat - same user was showing different colors in different contexts due to these class conflicts.

Solution

Remove bg-muted and text-foreground from AvatarFallback default classes. Components using AvatarFallback should always provide their own background and text colors via className prop.

Breaking Change

⚠️ Yes - AvatarFallback will no longer have a default background/text color.

Migration:

• All usage sites must provide className with background color
• For existing usage that relied on default, add bg-muted text-foreground to className
• AvatarGroup/AvatarGroupItem already passes colors, so no changes needed there

Test Plan

• ✅ Rebuilt package successfully
• ✅ Tested in LibreChat - avatar colors now consistent across all contexts
• Verify no other components in design system rely on default colors
• Update Storybook examples if needed

[robdimarco-atxp]

Closing in favor of a better approach - will fix AvatarGroupItem to properly separate container classes from fallback classes instead of removing default colors.