docs: lead the READMEs with a product demo gif

Replaces the Claude comparison gif at the top of all three READMEs with a
new one cut from the homepage demo. A visitor should see what the thing
does before being told it beats a competitor.

800x450 at 8fps to match the gif it replaces. Runs at real speed, not
accelerated — the comparison video's whole argument is that WebBrain is
faster, so a sped-up hero gif would undercut it and invite the obvious
objection.

Covers reading a page, filling a sign-up form, and fetching a file
including the "Approve & run" permission prompt. Drops the Bumble chapter,
and also the X chapter: at 19s it was the longest, and it detours into
searching X for mentions of WebBrain, which is filler inside a capability
demo. That brings it to 40s, close to the 37s of the previous gif.

5.1MB against the old 3.5MB. Not from the extra runtime — this footage has
far more motion than the mostly-static split screens in the comparison
video. This is the floor at 800x450: a 128-colour palette with dithering
off, which beat every bayer and sierra2_4a variant tested, some by more
than half. Going to 640x360 or 6fps would save another 0.7MB and looks
worse.

The comparison gif is deliberately kept in assets/ and still tracked. It
is only unreferenced, not deleted, so it can be dropped back in anywhere.

Author: esokullu

README.fr.md

@@ -23,7 +23,7 @@
   <a href="LICENSE">Licence MIT</a>
 </p>
 
-![Claude Chrome vs WebBrain](assets/webbrain-vs-claude-chrome.gif)
+![WebBrain lit une page, remplit un formulaire et télécharge un fichier](assets/webbrain-demo.gif)
 
 WebBrain est une extension de navigateur qui place un agent IA dans un panneau
 latéral, à côté de vos onglets. Posez-lui des questions sur la page où vous

README.md

@@ -23,7 +23,7 @@
   <a href="LICENSE">MIT License</a>
 </p>
 
-![Claude Chrome vs WebBrain](assets/webbrain-vs-claude-chrome.gif)
+![WebBrain reading a page, filling in a form, and fetching a file](assets/webbrain-demo.gif)
 
 WebBrain is a browser extension that puts an AI agent in a side panel next to
 your tabs. Ask it about the page you're on, or hand it a task and let it click,

README.zh-CN.md

@@ -23,7 +23,7 @@
   <a href="LICENSE">MIT 许可证</a>
 </p>
 
-![Claude Chrome vs WebBrain](assets/webbrain-vs-claude-chrome.gif)
+![WebBrain 阅读页面、填写表单并下载文件](assets/webbrain-demo.gif)
 
 WebBrain 是一个浏览器扩展，在标签页旁的侧边栏中放入一个 AI 智能体。你可以就当前页面
 向它提问，也可以交给它一项任务，让它自己点击、输入、导航完成。它运行在你选择的模型