docs(site): relocate GitHub stars to sidebar footer, add light/dark switcher (#294)

Move the live GitHub stars widget into the sidebar footer pill as a
type:"icon" link, sitting opposite the Slack mark (space-between) and
beside the theme switcher. Render it as inner content (GitHub mark +
star + compact count) so fumadocs supplies the anchor.

Replace the default fumadocs theme switcher with a custom two-icon
control where each icon selects its own theme. The built-in
"light-dark" mode is a single blind toggle that flips on any click, so
clicking the sun while already in light mode jumps to dark. useTheme is
sourced from fumadocs-ui/provider/base and the icons are inlined to
avoid bare next-themes / lucide-react imports.

docs-site/app/docs/layout.tsx

@@ -2,21 +2,10 @@ import { source } from "@/lib/source";
 import { DocsLayout } from "fumadocs-ui/layouts/docs";
 import type { ReactNode } from "react";
 import { baseOptions } from "@/app/layout.config";
-import { GitHubStars } from "@/components/github-stars";
 
 export default function Layout({ children }: { children: ReactNode }) {
   return (
-    <DocsLayout
-      tree={source.pageTree}
-      {...baseOptions}
-      sidebar={{
-        banner: (
-          <div className="flex">
-            <GitHubStars />
-          </div>
-        ),
-      }}
-    >
+    <DocsLayout tree={source.pageTree} {...baseOptions}>
       {children}
     </DocsLayout>
   );