feat: implement threaded replies and enhance feed layout with ThoughtThread component

thoughts-frontend/lib/utils.ts

@@ -1,6 +1,35 @@
 import { clsx, type ClassValue } from "clsx"
 import { twMerge } from "tailwind-merge"
+import { Thought } from "./api";
 
 export function cn(...inputs: ClassValue[]) {
   return twMerge(clsx(inputs))
 }
+
+export function buildThoughtThreads(allThoughts: Thought[]) {
+  const repliesByParentId = new Map<string, Thought[]>();
+  const topLevelThoughts: Thought[] = [];
+
+  // 1. Group all thoughts into top-level posts or replies
+  for (const thought of allThoughts) {
+    if (thought.replyToId) {
+      // It's a reply, group it with its parent
+      const replies = repliesByParentId.get(thought.replyToId) || [];
+      replies.push(thought);
+      repliesByParentId.set(thought.replyToId, replies);
+    } else {
+      // It's a top-level thought
+      topLevelThoughts.push(thought);
+    }
+  }
+
+  // 2. Sort top-level thoughts by date, newest first
+  topLevelThoughts.sort((a, b) => new Date(b.createdAt).getTime() - new Date(a.createdAt).getTime());
+
+  // 3. Sort replies within each thread by date, oldest first for conversational flow
+  for (const replies of repliesByParentId.values()) {
+    replies.sort((a, b) => new Date(a.createdAt).getTime() - new Date(b.createdAt).getTime());
+  }
+
+  return { topLevelThoughts, repliesByParentId };
+}