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

The issues are in the chat UI and debug console of the inspector. Look at the frontend components for the chat area, message input button, debug console display, and message badge rendering. The 'helloworld' sample agent from a2a-samples was used for testing. Check for existing tests related to these UI elements to understand the expected behavior and to verify changes.

Scritto dal modello di indicizzazione a partire dal testo della issue.