Design: the editor uses the design system, and its editor-* classes leave §5d #3635

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

Done:

• [ ] The file editor (e on a file) is drawn from §5's components (the src/web/ui.ts helpers and §5 classes) in place of .editor-mirror .editor-path .editor-preview .editor-stack .editor-tab .mirrored .ml (the 'the editor' bullet), which src/web/style/content.ts defines today.
• [ ] Those rules are gone and their bullet leaves docs/DESIGN.md §5d. bun scripts/design-lint.ts passes.
• [ ] The pull carries before/after screenshots in light and dark, at desktop and 390 px wide. Nothing a person reads or taps is lost, and keyboard use is unchanged.
• [ ] The part's tests pass, and a test finds none of the old class names in its HTML.

Files: src/web/style/content.ts, docs/DESIGN.md, the markup that uses the classes (git grep each), a test. Risk: a class borrowed elsewhere. git grep each name first, and keep any that is used elsewhere, said in the pull.

Parked by the crew: the check stayed red through 2 automatic tries (pull #3661 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.