🔥(frontend) remove custom DividerBlock

Blocknote now has a built-in divider block, so we
can remove our custom implementation.

src/frontend/apps/e2e/__tests__/app-impress/doc-export.spec.ts

@@ -274,49 +274,6 @@ test.describe('Doc Export', () => {
     expect(pdfData.text).toContain('Hello World'); // This is the pdf text
   });
 
-  /**
-   * We cannot assert the line break is visible in the pdf, but we can assert the
-   * line break is visible in the editor and that the pdf is generated.
-   */
-  test('it exports the doc with divider', async ({ page, browserName }) => {
-    const [randomDoc] = await createDoc(page, 'export-divider', browserName, 1);
-
-    const editor = page.locator('.ProseMirror');
-    await editor.click();
-    await editor.fill('Hello World');
-
-    // Trigger slash menu to show menu
-    await editor.locator('.bn-block-outer').last().fill('/');
-    await page.getByText('Add a horizontal line').click();
-
-    await expect(
-      editor.locator('.bn-block-content[data-content-type="divider"]'),
-    ).toBeVisible();
-
-    await page
-      .getByRole('button', {
-        name: 'Export the document',
-      })
-      .click();
-
-    await expect(
-      page.getByTestId('doc-open-modal-download-button'),
-    ).toBeVisible();
-
-    const downloadPromise = page.waitForEvent('download', (download) => {
-      return download.suggestedFilename().includes(`${randomDoc}.pdf`);
-    });
-
-    void page.getByTestId('doc-export-download-button').click();
-
-    const download = await downloadPromise;
-    expect(download.suggestedFilename()).toBe(`${randomDoc}.pdf`);
-
-    const pdfBuffer = await cs.toBuffer(await download.createReadStream());
-    const pdfData = await pdf(pdfBuffer);
-    expect(pdfData.text).toContain('Hello World');
-  });
-
   test('it exports the doc with multi columns', async ({
     page,
     browserName,