feat: add useTranscriptChat WebSocket hook

Task 5: Frontend WebSocket Hook
- Creates React hook for bidirectional chat WebSocket
- Handles token streaming with proper state accumulation
- Manages conversation history (user + assistant messages)
- Prevents memory leaks with isMounted check
- Proper cleanup on unmount
- Type-safe Message interface

Validated:
- No React dependency issues (removed currentStreamingText from deps)
- No stale closure bugs (using ref for streaming text)
- Proper mounted state tracking
- Lint passes with no errors
- TypeScript types correctly defined
- WebSocket cleanup on unmount

~100 lines

www/app/(app)/transcripts/useTranscriptChat.ts

@@ -0,0 +1,103 @@
+"use client";
+
+import { useEffect, useState, useRef } from "react";
+import { WEBSOCKET_URL } from "../../lib/apiClient";
+
+export type Message = {
+  id: string;
+  role: "user" | "assistant";
+  text: string;
+  timestamp: Date;
+};
+
+export type UseTranscriptChat = {
+  messages: Message[];
+  sendMessage: (text: string) => void;
+  isStreaming: boolean;
+  currentStreamingText: string;
+};
+
+export const useTranscriptChat = (transcriptId: string): UseTranscriptChat => {
+  const [messages, setMessages] = useState<Message[]>([]);
+  const [isStreaming, setIsStreaming] = useState(false);
+  const [currentStreamingText, setCurrentStreamingText] = useState("");
+  const wsRef = useRef<WebSocket | null>(null);
+  const streamingTextRef = useRef<string>("");
+  const isMountedRef = useRef<boolean>(true);
+
+  useEffect(() => {
+    isMountedRef.current = true;
+    const url = `${WEBSOCKET_URL}/v1/transcripts/${transcriptId}/chat`;
+    const ws = new WebSocket(url);
+    wsRef.current = ws;
+
+    ws.onopen = () => {
+      console.log("Chat WebSocket connected");
+    };
+
+    ws.onmessage = (event) => {
+      if (!isMountedRef.current) return;
+
+      const msg = JSON.parse(event.data);
+
+      switch (msg.type) {
+        case "token":
+          setIsStreaming(true);
+          streamingTextRef.current += msg.text;
+          setCurrentStreamingText(streamingTextRef.current);
+          break;
+
+        case "done":
+          setMessages((prev) => [
+            ...prev,
+            {
+              id: Date.now().toString(),
+              role: "assistant",
+              text: streamingTextRef.current,
+              timestamp: new Date(),
+            },
+          ]);
+          streamingTextRef.current = "";
+          setCurrentStreamingText("");
+          setIsStreaming(false);
+          break;
+
+        case "error":
+          console.error("Chat error:", msg.message);
+          setIsStreaming(false);
+          break;
+      }
+    };
+
+    ws.onerror = (error) => {
+      console.error("WebSocket error:", error);
+    };
+
+    ws.onclose = () => {
+      console.log("Chat WebSocket closed");
+    };
+
+    return () => {
+      isMountedRef.current = false;
+      ws.close();
+    };
+  }, [transcriptId]);
+
+  const sendMessage = (text: string) => {
+    if (!wsRef.current) return;
+
+    setMessages((prev) => [
+      ...prev,
+      {
+        id: Date.now().toString(),
+        role: "user",
+        text,
+        timestamp: new Date(),
+      },
+    ]);
+
+    wsRef.current.send(JSON.stringify({ type: "message", text }));
+  };
+
+  return { messages, sendMessage, isStreaming, currentStreamingText };
+};