The theme picker in the layout draws §5's field and button, and .theme-option, .theme-apply leave the markup #4700

Done:

• [ ] The theme picker in src/web/layout.ts draws each choice as a §5 field (radio with its label) and the apply button as a plain btn(); no theme-option or theme-apply in src/web outside src/web/style/.
• [ ] Choosing a theme still works without JavaScript (the form posts) and a test asserts the choices and the button.

Markup only: the stylesheet and docs/DESIGN.md are not touched in this pull (their rules leave in a follow-up), so it cannot conflict with the many pulls editing src/web/style/content.ts. Run bun x tsc --noEmit, bun scripts/design-lint.ts and bun test test/design-lint.test.ts before pushing; where counts fall, lower docs/design/baseline.json in the same pull.