feat: enhance user interface with improved styling and responsiveness

- Updated UserAvatar component to accept additional className for better customization.
- Refined ProfilePage layout with responsive avatar styling.
- Enhanced Header component with improved background and text styles.
- Improved PopularTags and TopFriends components with better spacing and text shadows.
- Updated ThoughtCard and ThoughtThread components for better visual hierarchy and responsiveness.
- Enhanced UI components (Button, Badge, Card, DropdownMenu, Input, Popover, Separator, Skeleton, Textarea) with new styles and effects.
- Added a new background image for visual enhancement.

thoughts-frontend/app/users/[username]/page.tsx

@@ -96,7 +96,11 @@ export default async function ProfilePage({ params }: ProfilePageProps) {
               <div className="flex justify-between items-start">
                 <div className="flex items-end gap-4">
                   <div className="w-24 h-24 rounded-full border-4 border-background shrink-0">
-                    <UserAvatar src={user.avatarUrl} alt={user.displayName} />
+                    <UserAvatar
+                      src={user.avatarUrl}
+                      alt={user.displayName}
+                      className="w-full h-full"
+                    />
                   </div>
                 </div>
                 {/* Action Button */}