Added Streaming Silent (Blind) Mode

### Describe the feature or problem you'd like to solve

Keep users focused by making their stream invisible

### Proposed solution

Stream history is not for review in most cases.

Command-confirmation has become more important.

Maintain decision-making quality by masking distracting llm stream

### Example prompts or workflows

1. Stream on/off can be controlled with slash commands.
2. After the command, all streams are hidden and an overlay screen is displayed.
3. The overlay screen has a Copilot logo in the center and hours, minutes, seconds at the bottom.
4. A heartbeat animation effect may be needed.
5. Add a shortcut key to dismiss the overlay while streaming.

### Additional context

The issue names no files, tests, or entry points. Start by locating the stream-rendering code, slash-command handling, and terminal shortcut processing, then determine how an overlay can cover output and track elapsed time. Done means the requested stream toggle, overlay, dismiss shortcut, and any heartbeat behavior are defined and tested.

由索引模型根据 Issue 内容生成。