feat(KB-077): add file editor textarea styles

- Add .file-editor-textarea CSS class with styling for file editing UI
- Include font family, sizing, and layout properties for monospace editing
- Add focus states and border styling for improved UX
- Integrate styles into dashboard stylesheet

packages/dashboard/app/styles.css

@@ -4431,6 +4431,30 @@ html .column.drag-over * {
   height: 100%;
 }
 
+/* Textarea-based file editor */
+.file-editor-textarea {
+  width: 100%;
+  min-width: 100%;
+  height: 100%;
+  background: var(--bg);
+  color: var(--text);
+  font-family: "SF Mono", Monaco, Consolas, monospace;
+  font-size: 14px;
+  line-height: 1.5;
+  padding: 16px;
+  border: none;
+  outline: none;
+  resize: none;
+  white-space: pre;
+  overflow-wrap: normal;
+  tab-size: 2;
+}
+
+.file-editor-textarea::selection {
+  background: var(--todo);
+  color: var(--bg);
+}
+
 /* File States */
 .file-error {
   padding: 12px 16px;