feat: implement Phase 3 container transformation with CLASSES.md compliance

- Add backend container transformation in engine.go following syntactic sugar specification
- Containers with .insertr get class removed and viable children get .insertr added
- Remove incorrect frontend container expansion - frontend only finds enhanced elements
- Fix StyleAwareEditor hasMultiPropertyElements runtime error
- Add addClass/removeClass methods to ContentEngine for class manipulation
- Update frontend to match HTML-first approach with no runtime container logic
- Test verified: container <section class='insertr'> transforms to individual h1.insertr, p.insertr, button.insertr

This completes the container expansion functionality per CLASSES.md:
Developer convenience (one .insertr enables section editing) + granular control (individual element editing)

demos/default/assets/style.css

@@ -211,6 +211,44 @@ body {
     margin-right: auto;
 }
 
+/* Default Link Formatting Styles */
+.link-primary {
+    color: #2563eb;
+    text-decoration: underline;
+    font-weight: 500;
+    transition: color 0.3s;
+}
+
+.link-primary:hover {
+    color: #1d4ed8;
+}
+
+.link-secondary {
+    color: #6b7280;
+    text-decoration: none;
+    border-bottom: 1px solid #d1d5db;
+    transition: border-color 0.3s;
+}
+
+.link-secondary:hover {
+    border-bottom-color: #374151;
+}
+
+.link-accent {
+    color: #7c3aed;
+    text-decoration: none;
+    background: linear-gradient(120deg, #7c3aed 0%, #7c3aed 100%);
+    background-repeat: no-repeat;
+    background-size: 100% 0.2em;
+    background-position: 0 88%;
+    transition: background-size 0.25s ease-in;
+}
+
+.link-accent:hover {
+    background-size: 100% 88%;
+    color: white;
+}
+
 /* Footer */
 .footer {
     background: #1f2937;