Port upstream soft break export fix

[Renakoni]

Summary

• Port upstream MarkText PR #4844 into the Android Muya copy.
• Preserve soft line breaks in exported HTML by rendering paragraph and tight-list content with white-space: pre-wrap.
• Add regression coverage that soft breaks remain conformant newlines instead of becoming <br>.

Upstream Source

• Upstream PR: fix(muya): keep soft line breaks when exporting to HTML/PDF marktext/marktext#4844
• Upstream issue: Line breaks (not paragraph breaks) ignored when exporting to PDF / HTML marktext/marktext#3676

Android Relevance

The Android repo carries Muya's HTML export implementation and stylesheet. A soft line break is represented as a bare newline inside a paragraph or tight list item. The live editor shows it because editor content uses pre-wrap behavior, but exported HTML could collapse it into a space. This keeps the embedded Muya export path consistent with editor rendering without changing the CommonMark HTML produced by getHighlightHtml().

Change Details

• Add export CSS for .markdown-body p and tight list items (li:not(:has(> p))) to use white-space: pre-wrap.
• Avoid applying that rule to loose list item containers so pretty-printed paragraph/list newlines do not become visible blank lines.
• Add softBreakExportHtml.spec.ts asserting soft breaks remain newlines and hard breaks still render as <br>.

Verification

• pnpm --dir third_party/muya exec vitest run src/state/__tests__/softBreakExportHtml.spec.ts passed.
• pnpm build passed.

[Renakoni]

Reviewed for the Android client: this fix lands entirely in Muya's HTML-export path (exportStyle.css, consumed only by MarkdownToHtml). The Android app has no code path that reaches exported HTML today — MarkdownToHtml appears only in an ambient type declaration, never called; sharing uses Markdown text/files and the live editor does not use the export stylesheet. Upstream, this serves the desktop export/print feature.

Recommendation: close as desktop-only surface for now. If an "export/share as HTML" feature ever lands on Android, this port can be revived from upstream #4844 — the change itself is a safe, CSS-only addition with a regression spec, so nothing blocks re-porting later.