Fix: Build error + explanation above exercise + aligned columns + impressum link

1. Removed stale 'explanations' export that broke build
2. Explanation banner now spans full width ABOVE the 2/3+1/3 layout
   so exercise cards and native words start at the same height
3. Both columns use items-start for visual alignment
4. Impressum link added at bottom of learn layout

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/app/learn/layout.tsx

@@ -32,6 +32,11 @@ export default function LearnLayout({ children }: { children: React.ReactNode })
           />
         </div>
         {children}
+        <div className="text-center py-4">
+          <a href="/impressum" className={`text-[10px] ${isDark ? 'text-white/20 hover:text-white/40' : 'text-slate-300 hover:text-slate-500'}`}>
+            Impressum
+          </a>
+        </div>
       </div>
     </div>
   )