Add DOM-level tests for the docs PR preview overlay (pr-preview.js)

Follow-up from Copilot's review of PR #9537 (FR-3879). The toolkit tests cover `diff:web` up to sidecar generation and HTML injection, but nothing executes the overlay (`packages/backend.ai-docs-toolkit/templates/assets/pr-preview.js`) in a DOM: mark placement, removed-block placeholders in list/table containers, popover modes (inline / side-by-side, formatting-only), image replacement + lightbox, viewed state (localStorage, fingerprints), the navigator, and keyboard shortcuts / focus handling. Add a browser or jsdom fixture (the toolkit already depends on Playwright for OG images) that loads a stamped page from a fixture build and exercises those paths. Manual verification was done in Chrome for #9537.

JIRA Issue: FR-3885

Start with packages/backend.ai-docs-toolkit/templates/assets/pr-preview.js and the existing toolkit tests covering diff:web through sidecar generation and HTML injection. Use the toolkit's existing Playwright dependency to load a stamped page from a fixture build. Done means DOM-level coverage for mark placement, removed-block placeholders, popover modes, image replacement and lightbox, viewed state, the navigator, and keyboard/focus handling.

Written by the indexing model from the issue text.