fix(docs): use light theme for Star History chart in both color schemes

Remove &theme=dark from the dark mode source so the chart always
renders with a light background regardless of GitHub's color scheme.

README.md

@@ -189,7 +189,7 @@ See [CONTRIBUTING.md](CONTRIBUTING.md) for the full development workflow, worktr
 
 <a href="https://www.star-history.com/?repos=multica-ai%2Fmultica&type=date&legend=bottom-right">
   <picture>
-    <source media="(prefers-color-scheme: dark)" srcset="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&theme=dark&legend=top-left" />
+    <source media="(prefers-color-scheme: dark)" srcset="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&legend=top-left" />
     <source media="(prefers-color-scheme: light)" srcset="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&legend=top-left" />
     <img alt="Star History Chart" src="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&legend=top-left" />
   </picture>

README.zh-CN.md

@@ -177,7 +177,7 @@ make start
 
 <a href="https://www.star-history.com/?repos=multica-ai%2Fmultica&type=date&legend=bottom-right">
   <picture>
-    <source media="(prefers-color-scheme: dark)" srcset="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&theme=dark&legend=top-left" />
+    <source media="(prefers-color-scheme: dark)" srcset="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&legend=top-left" />
     <source media="(prefers-color-scheme: light)" srcset="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&legend=top-left" />
     <img alt="Star History Chart" src="https://api.star-history.com/chart?repos=multica-ai/multica&type=date&legend=top-left" />
   </picture>