Add chat functionality between technician and client

- Add ChatMessage to protobuf definitions
- Server relays chat messages between agent and viewer
- Agent chat module shows messages via MessageBox
- Dashboard chat modal with WebSocket connection
- Simplified protobuf encoder/decoder in JavaScript

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

proto/guruconnect.proto

@@ -229,6 +229,17 @@ message LatencyReport {
     int32 bitrate_kbps = 3;
 }
 
+// ============================================================================
+// Chat Messages
+// ============================================================================
+
+message ChatMessage {
+    string id = 1;              // Unique message ID
+    string sender = 2;          // "technician" or "client"
+    string content = 3;         // Message text
+    int64 timestamp = 4;        // Unix timestamp
+}
+
 // ============================================================================
 // Control Messages
 // ============================================================================
@@ -282,5 +293,8 @@ message Message {
         Heartbeat heartbeat = 50;
         HeartbeatAck heartbeat_ack = 51;
         Disconnect disconnect = 52;
+
+        // Chat
+        ChatMessage chat_message = 60;
     }
 }