feat(website): menu adapts to the section beneath it (#22241)

## What

The sticky menu now adopts the color scheme of whatever section sits
beneath it as you scroll, across every page. Background, logo, and
buttons ease between schemes for a smooth handoff.

## How

- **Declarative contract** — sections opt in with a `data-menu-surface`
attribute (`SectionShell`, the footer, and the product hero's mobile
sections); `useActiveSurfaceScheme` tracks which surface sits under the
menu's bottom edge (`MENU_HEIGHT_PX`) and reports its `data-scheme`.
- **One provider** — `MenuStyleProvider` lives in the `(site)` layout so
every page adapts; the Menu resolves its scheme as `override ??
activeScheme ?? prop`. SSR seeds the prop to each page's first section,
so there's no mount flash.
- **Hero handoff** — the product hero keeps the menu via a per-frame
override only while its track still covers the nav band
(`controlsMenu`); it releases to the observer *before* the track clears
the bar, so the menu stays opaque on exit and never flashes the halftone
backdrop rising behind it. The menu's `backdrop-filter` was removed (a
no-op over the opaque menu, and a GPU compositing artifact).

## Also in this PR

- **Menu folder reorg** to the `product-feature` convention:
`components/`, `effect-components/`, `data/`, and `types/` (one domain
type per file).
- **`MENU_HEIGHT_PX` token** replacing the literal `64` that was
duplicated across four files (menu row, the hero's scroll model and
component, the observer).
- **`findActiveSurfaceScheme`** extracted as a pure, unit-tested
function (inclusive top / exclusive bottom, first-match, no-match,
null-scheme).
- **Mobile AI-section fix** — the mobile hero sections now declare the
surface contract, so the menu adapts dark over the AI block on mobile
(it previously stayed light). Their color is driven from `data-scheme`
(single source) rather than a parallel prop.

## Testing

- Jest — 16 tests (scroll model + surface-selection util).
- Headless Playwright (mobile 390px) — menu `light` over the intro →
`dark` over the AI section; section colors unchanged (`#fff` /
`rgb(20,20,20)`).
- Desktop unaffected — the mobile sections are `display:none` (zero
rect), so the observer skips them and the hero override path is
untouched.
- Gates green — typecheck, oxlint, oxfmt, check-conventions.

packages/twenty-website/src/ui/SectionShell.tsx

@@ -131,6 +131,7 @@ export function SectionShell({
       className={sectionShellClassName}
       data-connect-up={connectsUp ? '' : undefined}
       data-keep-top-rhythm={keepsTopRhythm ? '' : undefined}
+      data-menu-surface=""
       data-rhythm={rhythm}
       data-scheme={scheme}
     >