Add PokeDB.org data attribution footer to layout

Satisfies PokeDB.org's terms of use by adding a minimal attribution
footer linking to pokedb.org on every page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -127,6 +127,19 @@ export function Layout() {
       <main>
         <Outlet />
       </main>
+      <footer className="border-t border-gray-200 dark:border-gray-700 bg-white dark:bg-gray-800">
+        <div className="max-w-7xl mx-auto px-4 sm:px-6 lg:px-8 py-4 text-center text-xs text-gray-500 dark:text-gray-400">
+          Pokémon encounter data from{' '}
+          <a
+            href="https://pokedb.org"
+            className="underline hover:text-gray-700 dark:hover:text-gray-300"
+            target="_blank"
+            rel="noopener noreferrer"
+          >
+            PokeDB.org
+          </a>
+        </div>
+      </footer>
     </div>
   )
 }