Refine printed Markdown content rhythm

[PathGao]

Fixes #332.

Adds print-only CSS for:

• heading, paragraph, list, and blockquote paper spacing;
• code-block boundaries while allowing long code to flow across pages;
• image and table page-width constraints with basic spacing.

No interactive preview styles, printable table of contents, page headers/footers, or PDF renderer changes.

Validation:

• npm run check
• npm test (152 passing)
• cargo test --manifest-path src-tauri/Cargo.toml (27 passing)
• isolated macOS Markpad Print Test.app native print-preview smoke test with mixed Markdown fixture.

Depends on #331, continuing the linear chain.

[PathGao]

@alecdotdev 这是 #331 后的下一条链式 PR，范围按 #332 收窄为 print-only CSS：纸面间距、代码块分页边界、图片/表格页宽与留白。未引入独立打印 DOM、页眉页脚、目录或自定义 PDF 渲染。独立 macOS release bundle 已通过系统 print-preview 验证。