test(e2e): assert the empty sidebar with web-first checks

After deselecting every live category, the test polled
readVisibleSidebarCategoryNames() until it returned []. The poll
could hang when a row was removed mid-read (fixed generally in #1728).
Assert the empty state directly instead: no visible category rows via
toHaveCount(0), and the "No categories available" empty state in the
context panel.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/electron-backend-e2e/src/category-management.e2e.ts

@@ -448,11 +448,16 @@ test.describe('Electron Xtream Category Management', () => {
                 state: 'detached',
             });
 
-            await expect
-                .poll(() => readVisibleSidebarCategoryNames(app.mainWindow), {
-                    timeout: 15000,
-                })
-                .toEqual([]);
+            await expect(
+                app.mainWindow.locator(
+                    'app-workspace-context-panel .category-item:visible'
+                )
+            ).toHaveCount(0, { timeout: 15000 });
+            await expect(
+                app.mainWindow.locator(
+                    'app-workspace-context-panel .category-empty-state'
+                )
+            ).toContainText('No categories available');
 
             await expect(
                 app.mainWindow.getByRole('button', {