refactor(mobile): use separate explorer components for mobile (#8503)

close AF-1488

- remove dnd related logic
- separate styles
- remove empty status, always show a `New` button in each level of tree

tests/affine-mobile/e2e/utils.ts

@@ -13,3 +13,10 @@ export async function expandCollapsibleSection(page: Page, name: string) {
   await expect(section).toBeVisible();
   return section;
 }
+
+/**
+ * Click header "<" button
+ */
+export async function pageBack(page: Page) {
+  await page.getByTestId('page-header-back').tap();
+}