feat: add styles for drag & drop, preview panel, and undo/redo

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

styles.css

@@ -547,7 +547,7 @@
    ========================================================================== */
 
 .ff-builder-modal {
-  max-width: 700px;
+  max-width: 1100px;
   margin: 0 auto;
 }
 
@@ -565,6 +565,48 @@
   color: var(--text-normal);
 }
 
+/* --- Two-column layout --- */
+
+.ff-builder-layout {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 24px;
+  min-height: 300px;
+}
+
+.ff-builder-editor {
+  min-width: 0;
+}
+
+/* --- Preview panel --- */
+
+.ff-builder-preview {
+  border-left: 1px solid var(--background-modifier-border);
+  padding-left: 24px;
+  min-width: 0;
+}
+
+.ff-builder-preview h2 {
+  position: sticky;
+  top: 0;
+  background: var(--background-primary);
+  padding-bottom: 8px;
+  z-index: 1;
+}
+
+.ff-builder-preview-content {
+  max-height: 55vh;
+  overflow-y: auto;
+  padding-right: 4px;
+}
+
+.ff-builder-preview-empty {
+  padding: 32px 16px;
+  text-align: center;
+  font-size: 0.85rem;
+  color: var(--text-muted);
+}
+
 .ff-builder-general {
   margin-bottom: 8px;
 }
@@ -697,6 +739,34 @@
   padding: 10px;
 }
 
+/* --- Drag & Drop --- */
+
+.ff-drag-handle {
+  cursor: grab;
+  color: var(--text-faint);
+  font-size: 1rem;
+  line-height: 1;
+  user-select: none;
+  transition: color 0.15s ease;
+  padding: 0 2px;
+}
+
+.ff-drag-handle:hover {
+  color: var(--text-muted);
+}
+
+.ff-builder-field.ff-dragging {
+  opacity: 0.4;
+}
+
+.ff-builder-field.ff-drop-above {
+  border-top: 2px solid var(--interactive-accent);
+}
+
+.ff-builder-field.ff-drop-below {
+  border-bottom: 2px solid var(--interactive-accent);
+}
+
 /* --- Add field button --- */
 
 .ff-builder-add-btn {
@@ -724,13 +794,29 @@
 
 .ff-builder-footer {
   display: flex;
-  justify-content: flex-end;
+  justify-content: space-between;
+  align-items: center;
   gap: 8px;
   margin-top: 16px;
   padding-top: 12px;
   border-top: 1px solid var(--background-modifier-border);
 }
 
+.ff-builder-undo-redo {
+  display: flex;
+  gap: 4px;
+}
+
+.ff-builder-footer-right {
+  display: flex;
+  gap: 8px;
+}
+
+.ff-builder-action-btn:disabled {
+  opacity: 0.3;
+  cursor: not-allowed;
+}
+
 /* --- Settings tab empty state --- */
 
 .ff-settings-empty {