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/css/builder.css

@@ -5,10 +5,19 @@
 
 /* ── Page layout ─────────────────────────────────────────────── */
 
+#builder-page {
+    display: flex;
+    flex-direction: column;
+    height: 100vh;
+    overflow: hidden;
+}
+
 .builder-wrap {
     display: flex;
     flex-direction: column;
-    height: calc(100vh - 64px); /* below fixed admin header */
+    flex: 1;
+    min-height: 0; /* lets children shrink below their content height */
+    overflow: hidden;
 }
 
 .builder-toolbar {
@@ -131,6 +140,7 @@
     display: grid;
     grid-template-columns: 380px 1fr;
     flex: 1;
+    min-height: 0; /* required — without this the grid doesn't shrink in a flex column */
     overflow: hidden;
 }
 
@@ -140,6 +150,7 @@
     border-right: 1px solid var(--border);
     display: flex;
     flex-direction: column;
+    min-height: 0;
     overflow: hidden;
     background: var(--bg);
 }
@@ -294,6 +305,7 @@
 .builder-library {
     display: flex;
     flex-direction: column;
+    min-height: 0;
     overflow: hidden;
     background: var(--bg);
 }