Flowstate — Référence de design web et prompt

# Recreate this site as a single HTML file: Flowstate

You are an expert creative front-end developer. Produce a **single self-contained `index.html`**
that reproduces the project below **exactly** — same layout, visuals, motion, and interaction. Pure
HTML/CSS/JS in one file: no build step, no framework, no bundler. Use plain DOM and vanilla JS; the
only external dependency is the Google font (via a `<link>`) and the Lenis smooth-scroll library
(via a CDN ES-module import). Hardcode every value given here as a fixed constant. Rebuild each
component described below as a block of the one file.

## What it is

A full-viewport, immersive **dark hero** — a waitlist landing for a deep-work / focus app called