Make the top navigation always visible (same top nav on every page)

Split out of the #173 quick-wins thread as its own design issue.

## Problem
Doc pages and the landing page render different shells, so the top nav isn't consistently present across the site. Making every page show the same top nav as the landing page is worth doing on its own (it's a common complaint about the current navigation).

## Where it lives
`src/Layout.tsx` forks on `isLandingPage`:
- landing -> `LayoutLandingPage` with the full top nav
- doc pages -> the `doc-page` shell + `MenuModal`

So the work is unifying those branches: render the landing-style top nav on doc pages too.

## Why a separate issue
This is a layout/behavior change to the shell, not the CSS-var seam (#171 / the typography PR). It's bigger and design-sensitive, so worth agreeing on the target shell before building, rather than folding it into the quick-win stream.

Happy to prototype once there's a thumbs-up on the direction.

Start in src/Layout.tsx and compare the isLandingPage branches, including LayoutLandingPage and the doc-page shell with MenuModal. First confirm the intended shared shell and top-nav behavior with the issue discussion, then prototype the unified layout. Done means doc pages and the landing page consistently show the same top navigation.

Written by the indexing model from the issue text.