UserProfile missing avatar_thumbnail_url — uses full-size avatar everywhere

## Warning Backend UserProfileMe includes avatar_thumbnail_url: str|None for optimized thumbnails but frontend UserProfile type does not include it. ## Impact Frontend must use the full-size avatar_url for all contexts including small avatar circles in lists, cards, and comments. This is a performance concern — loading full-size images where thumbnails would suffice. ## Fix Add avatar_thumbnail_url: string|null to frontend UserProfile type and use it for small avatar displays. ## Found by API contract audit (2026-03-19)