fix: correct default sectionImage path in new-section layout

[dp-daesung]

Problem

The default value for sectionImage in layouts/new-section.vue is ../public/section-illustration.svg. Since this path does not start with /, resolveAssetUrl passes it through unchanged. The browser then tries to resolve it relative to the page URL, which fails — leaving the section slide with no illustration.

Reported in #8.

Root Cause: Regression in 1.1.2

In 1.1.1, the layout imported resolveAssetUrl from @slidev/client/layoutHelper, which handled the relative path correctly. PR #6 replaced this with a local ../layoutHelper, whose implementation only processes paths starting with /:

export function resolveAssetUrl(url: string) {
  if (url.startsWith('/'))
    return import.meta.env.BASE_URL + url.slice(1)
  return url  // ../public/... falls through here, broken in browser
}

The default value ../public/section-illustration.svg was never updated to match, causing it to break in 1.1.2.

How Slidev Serves Theme Public Assets

Slidev uses vite-plugin-static-copy to serve theme public/ assets under the /theme/ path (not /). So section-illustration.svg from the theme package is available at /theme/section-illustration.svg.

Fix

Change the default from ../public/section-illustration.svg to /theme/section-illustration.svg:

- default: "../public/section-illustration.svg",
+ default: "/theme/section-illustration.svg",

This allows sectionImage to be safely omitted in frontmatter and the illustration will render correctly.

Fixes #8