feat(modals): DOMContentLoaded bootstrap reopens modal from ?modal= URL

Refresh and share-link flows both work — the modal-state URL is the
canonical shareable artifact for "this overlay with this file open."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/l4d2web/static/js/modal-router.js

@@ -69,6 +69,13 @@
     dlg.addEventListener("click", (event) => {
       if (event.target === dlg) dlg.close();
     });
+
+    // Bootstrap: if the page loaded with ?modal=<path> already in the URL
+    // (refresh, share-link, or browser-history forward), open that modal.
+    const initialPath = new URL(window.location.href).searchParams.get("modal");
+    if (initialPath) {
+      fetchAndShow(initialPath);
+    }
   });
 
   // Browser back/forward: re-evaluate URL state and either fetch+show a new