a11y: Add skip to main content link for keyboard navigation

This adds a skip navigation link that allows keyboard users to bypass
the navigation and jump directly to the main content. The link is
visually hidden by default but becomes visible when focused.

This addresses WCAG 2.4.1 (Level A) - Bypass Blocks.

Related to #8360

Author: MorikawaSouma

src/components/Layout/Page.tsx

@@ -148,6 +148,12 @@ export function Page({
         routeTree={routeTree}
         breadcrumbs={breadcrumbs}
       />
+      {/* Skip to main content link for accessibility */}
+      <a
+        href="#main-content"
+        className="sr-only focus:not-sr-only focus:absolute focus:top-4 focus:left-4 focus:z-50 focus:bg-white focus:dark:bg-gray-900 focus:text-primary focus:dark:text-primary-dark focus:p-3 focus:rounded focus:border focus:border-border focus:dark:border-border-dark">
+        Skip to main content
+      </a>
       <div
         className={cn(
           hasColumns &&
@@ -166,7 +172,7 @@ export function Page({
         )}
         {/* No fallback UI so need to be careful not to suspend directly inside. */}
         <Suspense fallback={null}>
-          <main className="min-w-0 isolate">
+          <main id="main-content" className="min-w-0 isolate">
             <article
               className="font-normal break-words text-primary dark:text-primary-dark"
               key={asPath}>