Add preview GIF to README between Features and Installation sections

[Copilot]

Added a preview GIF animation to showcase the OSLogViewer functionality in the README.md file.

Changes Made

• Inserted preview.gif between the Features and Installation sections
• Added a new "功能预览" (Feature Preview) section header
• Used the specified markdown syntax: ![功能预览](preview.gif)
• Maintained proper spacing with blank lines above and below for clean structure
• Preserved all existing content without modification

Before

- 🧱 Built entirely with **SwiftUI**

## Installation

After

- 🧱 Built entirely with **SwiftUI**

## 功能预览

![功能预览](preview.gif)

## Installation

The preview GIF (7.6MB) was already present in the repository root directory and now provides users with a visual demonstration of the OSLogViewer's capabilities before they reach the installation instructions.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.