docs: add a dev container for Linux-consistent headed runs

[FRSgit]

One of the cheap items from the #212 list: instead of fighting macOS vs Linux font rendering, run cypress open inside Linux and review the diffs there.

This adds a .devcontainer/ folder that works for this repo and doubles as a copy-paste reference for anyone using the plugin. It's a single devcontainer.json, no Dockerfile:

• cypress/browsers:node-24.21.0-chrome-153.0.8010.36-1-ff-156.0-edge-153.0.4234.32-1 (same Node as .nvmrc, Chrome/Firefox/Edge preinstalled, amd64 + arm64)
• the desktop-lite feature, so you get a small Fluxbox desktop with noVNC on port 6080 and can click around in the Cypress app from your normal browser
• --shm-size=2g, because Chrome and Electron fall over on Docker's 64 MB default
• postCreateCommand does corepack + pnpm install + build + cypress verify

The README next to it explains why this exists, how to open it (VS Code, CLI, Codespaces), how to run pnpm test:e2e headed and see it, fonts (and the honest part: byte-identical output only happens if CI runs in the same image, there's a two-line container: snippet for that), Docker alternatives on macOS, Apple Silicon and --platform=linux/amd64, the Codespaces --shm-size caveat, how this pairs with the opt-in {platform} token for imagesPath (#422), and where it sits in the #212 plan: this is the full-fidelity "whole runner in Linux" option, the core path in RENDERER-DESIGN.md is a renderer sidecar that keeps Cypress native and headed and only moves the rendering into a pinned container, and the run-cypress CLI helper will be the scripted version of what this folder does by hand. The plugin README got a FAQ entry pointing at it and CONTRIBUTING mentions it as an optional way to work on the repo.

A couple of things I checked while writing this, since they change the config:

• the image runs as root and has no node user (it's built from a plain Debian trixie base, Node is installed by script), so remoteUser is root; the doc says how to add a user via common-utils if you care
• it sets CI=1 in the image env, which I unset in containerEnv so interactive tooling behaves normally
• it installs no fonts of its own beyond what the Chrome package pulls in, hence the fonts section

I built it here with the devcontainer CLI on Apple Silicon (arm64) and it came up fine: the desktop-lite feature installed, postCreateCommand ran through (pnpm 12.5.1, install, build, cypress verify against /root/.cache/Cypress/16.1.0), node -v is v24.21.0, DISPLAY is :1 and CI is unset in a fresh shell, the VNC/noVNC processes are running, and pnpm test:integration passes inside the container. What I didn't do is the headed part, so before or right after merging:

• pnpm test:e2e shows the Cypress app on port 6080 and the Batch Review carousel can approve an image into __image_snapshots_local__
• "Reopen in Container" also builds on an amd64 machine (the image is multi-arch, I only had arm64 at hand)

Refs: #212

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