feat: support multi-page editing and printing

[masonsxu]

Summary

• add multi-page editing with per-page markdown editors and previews
• fix browser printing so each preview page prints as its own A4 page
• update README usage instructions for multi-page workflow and printing

Motivation

The app currently focuses on fitting content into a single printable page. This feature expands the workflow to support multiple independently edited pages while keeping the preview and print output aligned.

Scope

• add page management in the editor UI
• render multiple A4 preview pages
• ensure print mode removes preview-only layout constraints
• document the updated usage flow in README

Acceptance Criteria

• users can add and remove pages in the editor UI
• each page has its own preview and fit calculation
• print preview shows all pages instead of only the first page
• printed pages are aligned correctly without right offset
• README explains basic usage, multi-page editing, and printing