docs: replace video with screenshot images for clean display

Use static demo screenshots instead of video embeds to avoid
GitHub's bordered video wrapper and sizing issues.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: JiaboLi-GitHub

README-CN.md

@@ -12,7 +12,7 @@ renderdoc-mcp 是一个基于 RenderDoc Replay API 的 MCP Server 和 CLI，提
 
 ## 演示
 
-<video src="https://github.com/JiaboLi-GitHub/renderdoc-mcp/raw/main/docs/demo/demo.mp4" controls muted width="100%"></video>
+![演示](docs/demo/demo.png)
 
 ## 功能概览
 

README.md

@@ -12,9 +12,7 @@ renderdoc-mcp is an MCP server and CLI that wraps the RenderDoc replay API into
 
 ## Demo
 
-
-https://github.com/user-attachments/assets/93854617-0c59-4eb6-b304-aa80bb3bb7d5
-
+![Demo](docs/demo/demo-en.png)
 
 ## Features