Add a headless-browser smoke test for the renderer, every layer type and MP4 export #14

The unit tests cover the storyboard schema, the server and every provider. The code that draws the video has no automated coverage at all:

• web/engine/render.mjs
• the 17 draw functions in web/engine/layers.mjs (only the pure helpers parseRuns and formatNumber are tested)
• web/engine/export.mjs

A layer that throws, like the negative-size lens/glass case in this backlog, only shows up when someone opens the studio.

The app already exposes a hook meant for exactly this, window.genmotion, with storyboard, seek(), exportMp4() and preloadAll(). Nothing uses it yet.

Task: add scripts/browser-check.mjs, run by npm run check:browser, that:

1. starts the server like scripts/check.mjs does
2. opens the studio in headless Chrome
3. loads a storyboard with one scene per LAYERS type, in each of 16:9, 9:16 and 1:1
4. seeks to several times per scene and asserts that no error was thrown
5. exports web/demo.genmotion.json and checks the MP4's size and duration

The project advertises zero dependencies, so keep the runtime free of them. Options: drive Chrome over the DevTools Protocol with Node's built-in WebSocket (Node 22+), or use playwright-core through npx without adding it to package.json. Explain the choice in the PR. A follow-up can wire this into CI.

Where: new scripts/browser-check.mjs; package.json:9-14; hook at web/app.mjs:657-658; web/engine/layers.mjs:480 (LAYER_DRAW); web/engine/storyboard.mjs:14-32 (LAYERS); server bootstrap to copy from scripts/check.mjs:1-30.

Getting started: Node 22, no npm install needed: git clone, then npm test and npm run check. Chrome or Edge for this one. Comment here to claim it. Questions → Discord.