UX issues observed during v1.0 testing

## Context

These UX issues were observed during end-to-end testing for the v1.0 protocol migration (PR #145). They are **pre-existing issues** unrelated to v1.0 changes, but worth tracking.

## Issues

### 1. Empty chat area before first message
The chat section shows a large blank void before any messages are sent. A placeholder like _"Send a message to start chatting with the agent"_ would improve the experience for first-time users.

### 2. Unlabeled "+" button next to message input
The `+` button next to the chat input has no tooltip or label. Its purpose (attach file? add metadata?) is unclear without clicking it.

### 3. Debug console only shows outbound requests
The debug console displays the JSON-RPC request sent to the agent, but does **not** show the agent's response. For a debugging tool, seeing both sides of the conversation is essential.

### 4. Error responses marked with ✅ compliant badge
When the agent returns an error (e.g., a transport or configuration failure), the error message in the chat is marked with a ✅ compliant badge. While the error may be _protocol-compliant_, displaying a green checkmark next to an error is confusing. Consider using a different indicator (e.g., ⚠️ or a neutral badge) for error responses that happen to be well-formed.

## Screenshots

Observed in headless Chromium (1280x900) testing against the `helloworld` sample agent from `a2a-samples`.

/cc #145