Design: a pull's inline review comments use §5's comment, and the rc-* classes leave §5d #3648

For: the design audit (#2678): one page part at a time onto §5's components, so docs/DESIGN.md §5d only shrinks.

Done:

• [ ] An inline review comment in the diff (its head, body, form, and its outdated and resolved states) is drawn with §5's comment component in place of .rc-body .rc-form .rc-head .rc-outdated .rc-resolved-line, which src/web/style/content.ts defines today.
• [ ] Those rules are gone and those five names leave the 'diff viewer' bullet of docs/DESIGN.md §5d (the bullet's other classes stay). bun scripts/design-lint.ts passes.
• [ ] The pull carries before/after screenshots of a pull's Files tab with an open, an outdated and a resolved comment, in light and dark, at desktop and 390 px wide.
• [ ] A test finds none of the five names in a Files page's HTML, and finds the comment's text and reply form.

Files: src/web/style/content.ts, docs/DESIGN.md, src/web/diff-view.ts (or whichever file renders them; git grep rc-head), a test. Risk: the reply form's keyboard use. Check that Tab order and Ctrl-Enter still work.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3654 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.