Prepare DocSprout v1.4.0 release

[ikelaiah]

Reader-quality improvements and v1.4.0 release preparation

This release fixes nested-page search links, makes search dismissal and recovery states consistent, preserves ordered-list starts and nested list structure, and keeps the skip link in WebKit's sequential tab order. It adds repeatable Chromium, Firefox and WebKit reader checks in CI with retained screenshots and logs, improves contributor and accessibility guidance, and updates the current version pins and release documentation to v1.4.0. Playwright remains development-only; Python runtime dependencies, schemas, generated routes and public CSS tokens are unchanged.

Local verification

• python -m unittest discover -s tests -t . -v — passed.
• ruff check . and node --check tests/browser_reader.mjs — passed.
• docsprout check, docsprout audit --strict and docsprout build --output build/release-review-site — passed; 41 pages, zero audit warnings/errors.
• Wheel and sdist built and passed artifact and installed-package qualification for v1.4.0.
• npm ci --offline — passed from the lockfile.
• Browser checks: Chromium 153 and Playwright WebKit 26.6 passed. Firefox tab subprocess creation fails in this Windows sandbox before DocSprout code runs (Error:-2147024809); the Ubuntu Firefox CI job must provide that qualification.

Review details and screenshot paths are in tasks/reader-quality-review.md.