feat(workspace): add line numbers and horizontal scrolling to code block editor

frontend/src/features/workspace/__tests__/CodeBlockEditor.test.tsx

@@ -209,4 +209,55 @@ describe('CodeBlockEditor', () => {
     expect(res2.updates[res2.updates.length - 1]?.type).toBe('code');
     expect(res2.updates[res2.updates.length - 1]?.lang).toBe('json');
   });
+
+  it('renders line numbers column with aria-hidden, user-select: none, and correct line count including trailing empty lines and 100+ lines', () => {
+    // 1. Single empty line
+    const { container, rerender } = render(
+      <CodeBlockEditor block={makeCodeBlock({ id: 'code-empty', content: '' })} content="" canEdit />
+    );
+    const getLineContainer = () => container.querySelector('[data-testid="code-line-numbers"]') as HTMLElement;
+    expect(getLineContainer()).toBeTruthy();
+    expect(getLineContainer().getAttribute('aria-hidden')).toBe('true');
+    expect(getLineContainer().className).toContain('select-none');
+
+    let lineEls = container.querySelectorAll('[data-line-number]');
+    expect(lineEls).toHaveLength(1);
+    expect(lineEls[0].textContent?.trim()).toBe('1');
+
+    // 2. Trailing empty line gives an extra line number (e.g. line 1 + empty line 2)
+    rerender(
+      <CodeBlockEditor
+        block={makeCodeBlock({ id: 'code-trailing', content: `first line
+` })}
+        content={`first line
+`}
+        canEdit
+      />
+    );
+    lineEls = container.querySelectorAll('[data-line-number]');
+    expect(lineEls).toHaveLength(2);
+    expect(lineEls[0].textContent?.trim()).toBe('1');
+    expect(lineEls[1].textContent?.trim()).toBe('2');
+
+    // 3. 120 lines (100+ lines test)
+    const longCode = Array.from({ length: 120 }, (_, i) => `const x_${i + 1} = ${i + 1};`).join('\n');
+    rerender(
+      <CodeBlockEditor
+        block={makeCodeBlock({ id: 'code-long', content: longCode })}
+        content={longCode}
+        canEdit
+      />
+    );
+    lineEls = container.querySelectorAll('[data-line-number]');
+    expect(lineEls).toHaveLength(120);
+    expect(lineEls[0].textContent?.trim()).toBe('1');
+    expect(lineEls[99].textContent?.trim()).toBe('100');
+    expect(lineEls[119].textContent?.trim()).toBe('120');
+
+    // Textarea has whitespace-pre and overlay has whitespace-pre
+    const textarea = container.querySelector('textarea') as HTMLTextAreaElement;
+    const pre = container.querySelector('pre') as HTMLPreElement;
+    expect(textarea.className).toContain('whitespace-pre');
+    expect(pre.className).toContain('whitespace-pre');
+  });
 });