fix: revert chat_stream initial content to empty string

The "..." text was unnecessary since the streaming bubble itself
shows bouncing dots via the frontend.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: recuu-pfeg

src/ws-server.ts

@@ -351,11 +351,11 @@ export class WsChatServer {
         // Save incoming message to API in background
         this.saveMessageToApi(msg.from ?? "user", "manager", msg.content).catch(() => {});
 
-        // Send immediate acknowledgment so user knows Manager is working
+        // Send streaming start indicator (creates the streaming bubble in UI)
         this.broadcast({
           type: WS_MSG.CHAT_STREAM,
           from: "manager",
-          content: "...",
+          content: "",
           timestamp: new Date().toISOString(),
         });