docs: replace README screenshot with demo video (#20)

* docs: replace screenshot with demo video

* docs: add README demo poster

* docs: use GitHub-hosted demo video

Author: jevin98

README.md

@@ -8,7 +8,7 @@
   <p>CSS-Diff is a DevTools sidebar extension for comparing the computed CSS of two DOM elements. It is built for frontend debugging: select two elements in the Elements panel, then inspect what changed, search for a property, and copy a CSS declaration with one click.</p>
 </div>
 
-![Screenshot](https://github.com/jevin98/css-diff-devtools/blob/main/img/screenshot.png?raw=true)
+https://github.com/user-attachments/assets/1766e65e-fa31-4095-93d1-f247daa0e8e7
 
 ## Why CSS-Diff?
 

README.zh-CN.md

@@ -8,7 +8,7 @@
   <p>CSS-Diff 是一个用于比较两个 DOM 元素计算后 CSS 的 DevTools 侧边栏扩展。它面向前端调试场景：在 Elements 面板中选择两个元素，即可查看哪些样式发生了变化、搜索指定属性，并一键复制 CSS 声明。</p>
 </div>
 
-![Screenshot](https://github.com/jevin98/css-diff-devtools/blob/main/img/screenshot.png?raw=true)
+https://github.com/user-attachments/assets/1766e65e-fa31-4095-93d1-f247daa0e8e7
 
 ## 为什么使用 CSS-Diff？