Port upstream 4844: keep soft line breaks on export via CSS

[Renakoni]

Fixes #143 — redoes the stale port from closed #66 against current main, matching the final upstream fix (marktext#4844, merged) exactly.

What changes

The editor shows a soft line break (a bare \n inside a paragraph) as a line break because .mu-content is white-space: pre-wrap, but HTML/PDF export collapsed it to a space — the two lines ran together.

Instead of forcing marked to emit <br> (which CommonMark reserves for hard breaks — the exported HTML would become non-conformant), the export keeps the conformant \n and renders it the way the editor does: white-space: pre-wrap on .markdown-body p and on tight list items (li:not(:has(> p))). Loose items are deliberately excluded — they wrap content in <p> (already covered), and li-level pre-wrap would expose marked's pretty-printing newline between </p> and </li> as a stray blank line.

The Android export path flows through the same vendored file: pdfExportHtml.ts → MarkdownToHtml.generate() → <article class="markdown-body"> with exportStyle.css inlined, so the one CSS rule covers both HTML and PDF export.

Verification

• New softBreakExportHtml.spec.ts (ported from upstream): paragraph soft break stays a newline and never becomes <br>; tight-item soft break likewise; a real hard break (two trailing spaces) still renders <br>.
• Muya app-gate suite: 1446/1446.
• Root unit tests (incl. the vendor-sync contract): 577/577; node_modules/@muyajs/core resynced.
• The rendered result of this exact rule (including the :has() loose-item exclusion) was verified in real Chromium upstream before marktext#4844 merged; the Android WebView shares the engine.