The Filter apply Cypress test has never passed

[HMarzban]

Problem

The apply test in apps/webapp/cypress/e2e/editor/filter/filter-apply-and-restore.cy.js has most likely never passed. It has two separate causes:

1. FilterPanel pushes /editor/<q>. Next routes that to the [...slugs] catch-all, and isReservedSlug('editor') returns notFound. editor became reserved in 7d8ca5257 (2026-06-15), and the spec landed in 30c2be918 (2026-06-17).
2. useApplyFilters runs only through useEditorAndProvider. EditorPlayground never mounts it, so .heading-fold-hidden never appears on the playground.

Fix

Pick one:

• Move the apply test to a real pad, or
• Skip it until the playground supports filtering.

Do not change the production push in FilterPanel to suit a test harness.

Related: #276.