.front-admin and .front-pick leave the stylesheet #5217

Intent: the front page's admin controls use §5's pieces. .front-admin (a centred max-width wrapper) and .front-pick (a row gap and an inline form margin) are each used once in src/web/studio/home.ts (near lines 416 and 561) and defined in src/web/style/content.ts:313-315.

Done:

• [ ] both draw from base.ts utilities or §5's row and bar, so the page looks the same at 360px and 1280px (before and after looks in the pull)
• [ ] the rules leave content.ts and the names leave docs/DESIGN.md §5d
• [ ] bun scripts/design-lint.ts and the home's tests pass

Fresh: replaces #5065. Both tries failed on tests it never touched (spawn-child-runs-apps; require_checks in apps.test.ts). This waits for those fixes.

duplicate of #5065