feat(web): chat UI with conversations, messages, and WebSocket streaming

Build the primary chat interface with conversation list sidebar,
message display area, streaming assistant responses via socket.io,
and auto-scrolling. Supports creating new conversations, sending
messages with optimistic updates, and real-time text streaming
from the agent via WebSocket events.

Components: ConversationList, MessageBubble, ChatInput, StreamingMessage
Libs: socket.io client singleton, shared types for Conversation/Message

Refs #28

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pnpm-lock.yaml

@@ -156,6 +156,9 @@ importers:
       react-dom:
         specifier: ^19.0.0
         version: 19.2.4(react@19.2.4)
+      socket.io-client:
+        specifier: ^4.8.0
+        version: 4.8.3
       tailwind-merge:
         specifier: ^3.5.0
         version: 3.5.0