fix: PDF image spacing, page top margin, and soft line breaks

[rogerdigital]

Problem

Exported PDFs had three layout defects, all stemming from how CSS interacts with Obsidian's rendered DOM:

1. Images stuck to page top (no top margin) on continuation pages
2. Excessive vertical spacing between consecutive images
3. Soft line breaks collapsed — adjacent text lines (no blank line in source) merged into one line

Root Causes & Fixes

Page top margin

Own physical margins via CSS @page with preferCSSPageSize: true, and zero the printToPDF margins. When both @page margin and printToPDF margins are non-zero, Chromium uses the printToPDF margins for the page box — and those drop the top margin to 0 on continuation pages. Verified via CDP Page.printToPDF (same path as Electron) + PyMuPDF coordinate measurement: with printToPDF margins at 0 and @page margin: 52px 60px, continuation-page top margin is correctly ~39pt.

Image spacing

Obsidian merges consecutive image embeds (and adjacent text) into a single <p> separated by <br>. Broadened the media-paragraph spacing collapse from :has(> img:only-child) to descendant :has(img) so multi-media paragraphs tighten. Media gap is controlled by line-height: 1 (shrinks the <br> line-box) plus per-media margin.

Soft line breaks

A prior rule p:has(img) > br { display:none } — meant to tighten media spacing — collapsed all <br> in media paragraphs, silently merging legitimate soft line breaks when text and media shared one <p>. Removed the rule entirely. Added a regression test forbidding display:none on <br> anywhere in the PDF stylesheet.

Verification

• 159 tests pass
• PyMuPDF coordinate measurement confirms all soft-break paragraphs render on separate lines
• Other formats audited: html-document, docx, markdown-bundle all handle soft breaks correctly (no change needed)