fix: html-router modals and add insta snapshot tests.

Avoid nested forms in the scratchpad editor, centralize modal lifecycle in modal.js, return HTMX partials from archive, and add template compile plus layout snapshots.

html-router/assets/admin-prompt-reset.js

@@ -0,0 +1,19 @@
+/**
+ * Shared "Reset to Default" handler for the admin prompt-edit modals
+ * (templates/admin/edit_*_prompt_modal.html).
+ *
+ * Each reset button carries data-reset-target with a selector for the prompt
+ * textarea to repopulate from the modal's hidden #default_prompt_content.
+ */
+(function () {
+  'use strict';
+
+  document.body.addEventListener('click', function (e) {
+    const btn = e.target.closest('[data-reset-target]');
+    if (!btn) return;
+    const scope = btn.closest('dialog') || document;
+    const source = scope.querySelector('#default_prompt_content');
+    const target = scope.querySelector(btn.dataset.resetTarget);
+    if (source && target) target.value = source.value;
+  });
+})();