This introduces a long-awaited mobile view for the diff viewer. This is

a pure-CSS solution for viewing existing diffs in a mobile, unified diff

display, without the need for a re-render of a diff.
All diff functionality works as normally. Users can browse the diff,

expand, collapse, download files, make multi-line comments,

view/navigate moved line flags, and view interdiffs (though the revision

slider has interaction problems on mobile).
On mobile, the diff viewer is rendered as a CSS Grid, with three

columns: Original line number, modified line number, and line content.
The grid's column setup provides ample room for large line numbers

(safely up to 6 digits) before a given row's line number column

truncates (it will truncate on the left). This is for browsers without

support for Subgrids. For those that do support Subgrids, the columns

will resize for the number of digits needed (and reserve less space to

start).
Inserts, deletes, and equals present the same as before, though only one

column of line content is shown. Replaces work differently, as they're

shown in pairs of deletes and inserts, helping visualize on a

line-by-line basis how the content has changed.
This change doesn't require any JavaScript or server-side work.
There are some queries using the modern :has() selector, which Firefox

doesn't fully support. This is just for cleaner border styling, and is

not detrimental. Firefox will have improved styling once this is fully

supported (which, according to bug reports, should be any day now).

Tested on Chrome, Firefox, and Safari with and witout Subgrids.
Tested the diff viewer on both desktop and mobile views.
On mobile, I tested move detection, commenting (single- and multi-line),

file downloads, move flags, expansion/collapsing, and interdiffs, with

all kinds of mixes of inserts/deletes/replaces.