fix(builder): emit Container cssId/cssClass with live panel controls (D1a)

Container.craft.props collected cssId/cssClass since before Phase E1 but
nothing rendered or exported them. Add live "CSS ID" / "CSS Class" text
inputs to ContainerStylePanel (guarded on nodeProps.cssId/cssClass
!== undefined) and emit id=/class= in both the editor render and toHtml.
cssId takes precedence over the existing anchorId prop when both are set
(only one id attribute can be emitted); anchorId is used as a fallback.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/panels/right/styles/ContainerStylePanel.tsx

@@ -11,6 +11,9 @@ import {
   ColorSwatchGrid,
   GradientSwatchGrid,
   PresetButtonGrid,
+  labelStyle,
+  inputStyle,
+  sectionGap,
 } from './shared';
 
 /* ---------- CONTAINER / SECTION ---------- */
@@ -27,8 +30,39 @@ export const ContainerStylePanel: React.FC<StylePanelProps> = ({ selectedId, nod
     [actions, selectedId],
   );
 
+  const setProp = useCallback(
+    (key: string, value: string) => {
+      actions.setProp(selectedId, (props: any) => { props[key] = value; });
+    },
+    [actions, selectedId],
+  );
+
   return (
     <>
+      {nodeProps.cssId !== undefined && (
+        <div style={sectionGap}>
+          <label style={labelStyle}>CSS ID</label>
+          <input
+            type="text"
+            value={nodeProps.cssId || ''}
+            onChange={(e) => setProp('cssId', e.target.value)}
+            placeholder="my-element-id"
+            style={inputStyle}
+          />
+        </div>
+      )}
+      {nodeProps.cssClass !== undefined && (
+        <div style={sectionGap}>
+          <label style={labelStyle}>CSS Class</label>
+          <input
+            type="text"
+            value={nodeProps.cssClass || ''}
+            onChange={(e) => setProp('cssClass', e.target.value)}
+            placeholder="my-custom-class"
+            style={inputStyle}
+          />
+        </div>
+      )}
       <div className="guided-section">
         <SectionLabel>Background Color</SectionLabel>
         <ColorSwatchGrid