fix(oc-docs): unify topbar control heights and convert sizing to rem

[sundram-bruno]

Aligns the topbar controls to a uniform height and moves their sizing to rem.

What changed (behaviour)

• Topbar controls now share a uniform 1.75rem (28px) height: theme toggle, env switcher, Open-in-Bruno, and the search bar (plus the hamburger/search icon buttons that were already 1.75rem / 28px).
• Theme toggle grew from 1.625rem (26px) to 1.75rem (28px) square.
• Env switcher box grew from ~1.625rem (~26px) to 1.75rem (28px) (fixed height replaces the vertical padding).
• Open-in-Bruno is visually unchanged (kept at 1.75rem / 28px, weight 600, 1rem / 16px glyph).
• Search bar is visually unchanged (already 1.75rem / 28px).

Non-behavioural

• Converted px sizing to rem across Open-in-Bruno, theme toggle, env switcher, show-vars toggle, and the search bar. Same rendered size at the default 16px root, but now scales with the user's font size.
• Show-vars toggle: pure px to rem, no size change.
• Left as px on purpose: the env-switcher (max-width: 640px) and show-vars (max-width: 900px) media-query breakpoints, and the show-vars toggle track's 999px pill radius.

[vasharma05-bruno]

LGTM, thanks!