fix: center wide docs content

[Si40Code]

Summary

• center the article and table-of-contents columns as one content group on wide screens
• preserve the 42rem reading width and the framework's 32px column gap
• add a 1920px regression test that rejects both unbalanced trailing whitespace and an oversized middle gap

Root cause

The custom Markdown layout forced justify-content: start inside a 96rem canvas. On a 1920px viewport, that left 528px of unused space after the table of contents. Stretching the columns apart only moved that empty space into a 560px middle gap, so the final layout centers the two columns together instead.

Impact and compatibility

• wide documentation pages now have balanced outer whitespace and keep related navigation close to the article
• narrow-screen behavior is unchanged
• no backend, OpenAPI, SDK, guide-content, or changelog contract changes
• backward compatible

Validation

• pnpm run validate
• TypeScript typecheck passed
• static build and distribution verification passed
• deployment script tests: 11 passed
• Playwright tests: 18 passed
• visual verification at 1920×1080: 32px article/TOC gap and 264px balanced space on each side of the content group

Screenshot

The approved 1920×1080 local preview is attached below.