feat(modals): click intercept + openModal + fetchAndShow

a[data-modal] clicks push ?modal=<path> to URL and trigger htmx.ajax
into #modal-content with the HX-Modal header. window.openModal exposed
for non-<a> trigger sites (files-overlay row clicks). Race guard via
currentModalPath token. Close/popstate/bootstrap follow.

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

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

@@ -1,6 +1,51 @@
-// URL-addressable modal router (see docs/superpowers/specs/2026-05-17-url-addressable-modals-design.md).
+// URL-addressable modal router (see spec 2026-05-17-url-addressable-modals).
-// Implementation lands in subsequent tasks; this stub keeps base.html's
+// Click intercept on a[data-modal] → ?modal=<path> in URL → htmx swap into
-// script include from 404'ing during the staged rollout.
+// #modal-content → showModal(). Close/popstate/bootstrap in later tasks.
 (function () {
   "use strict";
+
+  let currentModalPath = null;  // race-guard against stale swaps
+
+  function openModal(path) {
+    const url = new URL(window.location.href);
+    url.searchParams.set("modal", path);
+    history.pushState({ modal: path }, "", url.toString());
+    fetchAndShow(path);
+  }
+
+  function fetchAndShow(path) {
+    currentModalPath = path;
+    if (typeof window.htmx === "undefined") {
+      console.error("[modal-router] htmx not loaded; cannot fetch modal");
+      return;
+    }
+    window.htmx.ajax("GET", path, {
+      target: "#modal-content",
+      swap: "innerHTML",
+      headers: { "HX-Modal": "1" },
+    }).then(() => {
+      // Race guard: if the user clicked again during the fetch, abandon
+      // this swap; the newer click will win.
+      if (currentModalPath !== path) return;
+      const dlg = document.getElementById("modal-container");
+      if (dlg && !dlg.open) dlg.showModal();
+    }).catch((err) => {
+      console.error("[modal-router] fetch failed", err);
+    });
+  }
+
+  document.addEventListener("click", (event) => {
+    const link = event.target.closest("a[data-modal]");
+    if (!link) return;
+    if (event.metaKey || event.ctrlKey || event.shiftKey || event.altKey) return;
+    if (event.button !== 0) return;
+    const href = link.getAttribute("href");
+    if (!href) return;
+    event.preventDefault();
+    openModal(href);
+  });
+
+  // Public API — used by files-overlay.js to open the editor from row clicks
+  // that aren't a literal <a data-modal> (existing event delegation).
+  window.openModal = openModal;
 })();