Add source link footer pointing to GitHub repo

src/chess_pressure/static/index.html

@@ -102,6 +102,10 @@
     </form>
   </dialog>
 
+  <footer>
+    <a href="https://github.com/mindthemath/chess-pressure">source</a>
+  </footer>
+
   <script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
   <script src="https://unpkg.com/@chrisoakman/chessboardjs@1.0.0/dist/chessboard-1.0.0.min.js"></script>
   <script src="https://cdnjs.cloudflare.com/ajax/libs/chess.js/0.10.3/chess.min.js"></script>

src/chess_pressure/static/style.css

@@ -328,6 +328,18 @@ dialog textarea {
 }
 .dialog-buttons button:hover { background: var(--move-active); }
 
+/* --- Footer --- */
+footer {
+  text-align: center;
+  padding: 1rem 0;
+  font-size: 0.75rem;
+}
+footer a {
+  color: var(--fg2);
+  text-decoration: none;
+}
+footer a:hover { text-decoration: underline; }
+
 /* --- Mobile --- */
 @media (max-width: 720px) {
   main {