[codex] Fix mobile navbar search overlap

[nytera]

What changed

• keep the mobile navbar search container in the flex flow on production builds
• reserve stable width for the theme toggle so it stays clickable on narrow screens
• extend the browser regression test to assert that mobile search does not overlap the theme toggle at 320px

Why

The generated navbar search container could end up position: absolute in the production CSS order, which let the search trigger sit on top of the color mode toggle on mobile.

Impact

• mobile navbar search and theme toggle no longer visually overlap
• the regression is now covered in Playwright against the built site

Validation

• npm test -- src/theme/SearchBar/index.test.tsx
• npm run build
• npx playwright test e2e/navbar-search.spec.ts