Polish workbench shell density

frontend/src/App.tsx

@@ -452,6 +452,9 @@ function App(): JSX.Element {
 
   return (
     <div className="app-shell workbench-shell">
+      <a className="skip-link" href="#workspace-main">
+        Skip to workspace
+      </a>
       <header className="workbench-topbar">
         <div className="brand-block">
           <p className="eyebrow">GeoAI Workbench</p>
@@ -481,7 +484,7 @@ function App(): JSX.Element {
 
       <div className="workbench-layout">
         <aside className="workbench-sidebar" aria-label="Workbench navigation">
-          <nav>
+          <nav aria-label="Primary workspaces">
             {workspaceNavItems.map((item) => (
               <button
                 key={item.key}
@@ -499,7 +502,7 @@ function App(): JSX.Element {
           </nav>
         </aside>
 
-        <main className="workbench-main">
+        <main className="workbench-main" id="workspace-main" tabIndex={-1}>
           <div className="workspace-heading">
             <div>
               <p className="eyebrow">Workspace</p>