feat: add avatars to comments (web + iOS) with replies, delete, sort, and crop fix

- Batch-resolve avatar presign URLs server-side in GET /api/comments/[slug];
  returns avatarUrls map alongside comments and myVotes
- CommentsSection.svelte: show avatar image or initials fallback (24px top-level,
  20px replies) next to each comment/reply username
- iOS CommentsResponse gains avatarUrls field; CommentsViewModel stores and
  populates it on load; CommentRow renders AsyncImage with initials fallback
- Also includes: comment replies (1-level nesting), delete, sort (Top/New),
  parent_id schema migration, and AvatarCropModal cropperjs fix

scripts/pb-init.sh

@@ -213,7 +213,8 @@ create_collection "book_comments" '{
     {"name": "body",       "type": "text",   "required": true},
     {"name": "upvotes",    "type": "number"},
     {"name": "downvotes",  "type": "number"},
-    {"name": "created",    "type": "date"}
+    {"name": "created",    "type": "date"},
+    {"name": "parent_id",  "type": "text"}
   ]
 }'