Redesign community section with social media feed UX; v1.2.6

- Twitter/X-style post cards: avatar thread line, author + relative timestamp
- Heart like button with optimistic toggle and like count per post
- Comment button with reply count; inline threaded replies always visible
- Compose card with user avatar, borderless textarea, pill Post button
- Horizontal scrollable lesson filter chips (All + per-lesson)
- New POST /api/study-community/posts/:postId/like endpoint (toggle)
- GET community posts now returns likedByMe + likeCount per post
- sanitizeStudyCommunityPosts preserves likedBy array on disk
- Relative timestamps (just now, Xm ago, Xh ago, Xd ago, then date)
- UserAvatar helper renders img or initials fallback consistently

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/data.js

@@ -1523,6 +1523,7 @@ export function sanitizeStudyCommunityPosts(value) {
         ? item.replies.map(sanitizeStudyCommunityReply).filter(Boolean).slice(0, 50)
         : []
       if (!studySlug || !message) return null
+      const likedBy = Array.isArray(item.likedBy) ? item.likedBy.filter(id => typeof id === 'string' && id.trim()).slice(0, 1000) : []
       return {
         id: typeof item.id === 'string' && item.id.trim() ? item.id.trim() : randomUUID(),
         studySlug,
@@ -1532,6 +1533,7 @@ export function sanitizeStudyCommunityPosts(value) {
         message,
         createdAt: typeof item.createdAt === 'string' ? item.createdAt : new Date().toISOString(),
         replies,
+        likedBy,
       }
     })
     .filter(Boolean)