Serve delete, move and merge dialogs as htmx fragments

The server renders the dialogs (?dialog=delete|move) and answers a move
that needs confirmation with the merge dialog itself, so the client no
longer builds action URLs or interprets X-Merge-Available. Successful
mutations send HX-Redirect, which hx.js turns into a history replace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

assets/hx.js

@@ -0,0 +1,30 @@
+// Site-wide htmx policy. Loaded right after htmx itself.
+(function () {
+    // htmx wires up swapped-in content (e.g. a dialog's hx-post form) in its
+    // settle step, by default 20ms after the swap: a click or Enter in that
+    // window would submit the form natively. No settle transitions are used,
+    // so settle in the same tick.
+    htmx.config.defaultSettleDelay = 0;
+
+    // A server-sent HX-Redirect follows a mutation (move, delete, …): load the
+    // target into the current history entry rather than pushing a new one, so
+    // Back skips the stale pre-mutation page. replaceState + reload rather than
+    // location.replace: a target differing only by #fragment would otherwise
+    // not re-fetch.
+    htmx.location = {
+        set href(target) {
+            window.history.replaceState(null, '', target);
+            window.location.reload();
+        },
+        reload: function () { window.location.reload(); }
+    };
+
+    // Failed requests answer with a plain-text reason; show it.
+    document.addEventListener('htmx:responseError', function (e) {
+        var xhr = e.detail.xhr;
+        alert(xhr.responseText || ('Request failed (' + xhr.status + ')'));
+    });
+    document.addEventListener('htmx:sendError', function () {
+        alert('Network error');
+    });
+})();