feat(KB-226): add markdown preview to FileEditor

- Add markdown preview toggle to FileEditor with live preview pane
- Implement syntax-highlighted code blocks and responsive layout
- Add comprehensive tests for preview toggle and content rendering
- Update dashboard README with FileEditor usage documentation
- Add changeset for markdown preview feature
- Clean up obsolete changeset files

packages/dashboard/app/styles.css

@@ -6194,6 +6194,8 @@ html .column.drag-over * {
 /* File Editor Container */
 .file-editor-container {
   flex: 1;
+  display: flex;
+  flex-direction: column;
   overflow: hidden;
   background: var(--bg);
 }
@@ -6202,6 +6204,37 @@ html .column.drag-over * {
   height: 100%;
 }
 
+/* File Editor Toolbar */
+.file-editor-toolbar {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  padding: 8px 16px;
+  border-bottom: 1px solid var(--border);
+  background: var(--surface);
+  flex-shrink: 0;
+}
+
+.file-editor-mode-toggle {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+}
+
+.file-editor-mode-toggle .btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+}
+
+/* File Editor Preview */
+.file-editor-preview {
+  flex: 1;
+  overflow-y: auto;
+  padding: 16px;
+  min-height: 0;
+}
+
 /* Textarea-based file editor */
 .file-editor-textarea {
   width: 100%;
@@ -6438,7 +6471,9 @@ html .column.drag-over * {
 .file-browser-list {
   flex: 1;
   overflow-y: auto;
+  overflow-x: hidden;
   padding: 4px 0;
+  min-height: 0;
 }
 
 .file-browser-empty {