feat(workspace): add external file change detection, conflict resolution UI, and diff editor

ts_web/elements/00group-workspace/dees-workspace-monaco/dees-workspace-monaco.ts

@@ -242,4 +242,53 @@ export class DeesWorkspaceMonaco extends DeesElement {
       this.monacoThemeSubscription = null;
     }
   }
+
+  /**
+   * Update content from external source with optional cursor preservation.
+   * Use this when the file content changes externally (e.g., file changed on disk).
+   * @param newContent The new content to set
+   * @param preserveCursor Whether to preserve cursor/scroll position (default: true)
+   */
+  public async setContentExternal(
+    newContent: string,
+    preserveCursor: boolean = true
+  ): Promise<void> {
+    const editor = await this.editorDeferred.promise;
+    const currentValue = editor.getValue();
+
+    if (currentValue === newContent) return;
+
+    // Save cursor state if preserving
+    const position = preserveCursor ? editor.getPosition() : null;
+    const selections = preserveCursor ? editor.getSelections() : null;
+    const scrollTop = preserveCursor ? editor.getScrollTop() : 0;
+    const scrollLeft = preserveCursor ? editor.getScrollLeft() : 0;
+
+    // Update content
+    this.isUpdatingFromExternal = true;
+    editor.setValue(newContent);
+    this.isUpdatingFromExternal = false;
+
+    // Restore cursor state if preserving
+    if (preserveCursor) {
+      if (position) {
+        // Clamp position to valid range
+        const model = editor.getModel();
+        const lineCount = model?.getLineCount() || 1;
+        const clampedLine = Math.min(position.lineNumber, lineCount);
+        const lineLength = model?.getLineMaxColumn(clampedLine) || 1;
+        const clampedColumn = Math.min(position.column, lineLength);
+        editor.setPosition({ lineNumber: clampedLine, column: clampedColumn });
+      }
+      if (selections && selections.length > 0) {
+        // Selections may be invalid after content change, wrap in try-catch
+        try {
+          editor.setSelections(selections);
+        } catch {
+          // Ignore invalid selections
+        }
+      }
+      editor.setScrollPosition({ scrollTop, scrollLeft });
+    }
+  }
 }