Fix Builder layout and Loading... initialization bug

Layout: replace admin-container wrapper with #builder-page flex column;
add min-height:0 to builder-wrap, builder-body, both panels so the
inner scroll areas actually shrink correctly in a flex column.

Loading...: scripts at end of body mean DOMContentLoaded may have
already fired by the time the listener is registered. Check
document.readyState first and call init() directly if DOM is ready.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/js/builder.js

@@ -17,7 +17,7 @@
     /* ─────────────────────────────────────────────────────────
        Boot
     ───────────────────────────────────────────────────────── */
-    document.addEventListener('DOMContentLoaded', function () {
+    function init() {
         PRAYERS = window.PRAYERS_DATA || [];
 
         // Populate existing steps when editing a session
@@ -31,7 +31,15 @@
         renderLibrary();
         renderSequence();
         bindEvents();
-    });
+    }
+
+    // Scripts are at end of body — DOM is fully parsed by the time this runs,
+    // so DOMContentLoaded may have already fired. Check readyState first.
+    if (document.readyState === 'loading') {
+        document.addEventListener('DOMContentLoaded', init);
+    } else {
+        init();
+    }
 
     /* ─────────────────────────────────────────────────────────
        Event bindings