#86 diff: block rows separated by newlines render a blank line between every row

What

diff_html emits block-level rows separated by literal \n inside <pre>:

</span>\n<span class="df-line" ...>

pre preserves whitespace, so each newline creates an extra empty line box between rows. Every diff row is followed by a blank row — the diff looks striped and is twice as tall.

Reproduced

/commit/<sha>.html and PR Patch sections, desktop and mobile. HTML confirms </span>\n<span class="df-line"; screenshot shows green/pink bands with white gaps equal to line height.

Impact

Poor readability of the core review surface; mobile diffs get very long.

Fix

Emit no newline between block rows (or make rows inline-block and keep newlines; but then full-width backgrounds need attention). Keep the newlines out of the <pre> and let each row be a block. Add a test asserting no </span>\n<span class="df-line" and a visual check.