Design: the editor's tabs, path and preview use the design system, and the editor-* classes leave §5d #3874

For: the design audit (#2678): one page at a time onto §5's components, so docs/DESIGN.md §5d only shrinks. The editor is the developer depth one step from a post; it should look like the forge, not a separate tool.

Done:

• [ ] The editor page's tabs, path, preview and stack is drawn from §5's components (src/web/ui.ts helpers such as row, card, badge, btn, and §5 classes) in place of .editor-mirror .editor-path .editor-preview .editor-stack .editor-tab, which src/web/style/content.ts defines today. src/web/client.ts selects some of these names: move its selectors to data-* attributes in the same pull, so the live behaviour is kept. .mirrored and .ml may stay if the editor's mirror needs them; say so in the pull.
• [ ] Those classes' rules are gone, and their names leave their bullet of docs/DESIGN.md §5d. bun scripts/design-lint.ts passes, and so does --update with a lower or equal baseline.
• [ ] 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 every form posts the same fields.
• [ ] The page's existing tests pass. A test asserts the old class names are absent from the page's HTML.

Files: src/web/pages/edit.ts, src/web/client.ts, src/web/style/content.ts, docs/DESIGN.md, the page's test. Risk: another page borrowing one of these classes. git grep each name first, and if one is used elsewhere, keep it and say so in the pull.

Done on main already, waiting for you to close it: 018aac7f4649119ad47788bed382eded945a392a

Closed: already done on main in 018aac7. Closed by the crew's live triage; reopen it to undo, and it stays open.