feat(dev-preview): local browser preview for css/js with live reload

[Eetwalt]

Prototype to simplify the proxy setup to one npm command and no need to set up certs. With limited testing, seems to work nicely. Not that battle-tested yet, but wanted to share if you want to try it out while I'm out for the next couple of weeks!

Summary

• Add scripts/dev-preview/ and an npm run dev command: a local MITM proxy serves your locally-built CSS/JS in place of production assets on *.liquipedia.net/commons/load.php, a file watcher rebuilds on .scss/.js save, and a dedicated browser auto-reloads — no editor integration required.
• Node-native (http-mitm-proxy + chokidar), no Python/venv. Proxying is PAC-scoped to *.liquipedia.net and runs in a throwaway browser profile that ignores cert errors, so normal browsing is untouched.
• Previews CSS/JS only — Lua renders server-side and still needs the dev-wiki deploy flow (noted in the README).
• Complements the minimal mitmproxy + VSCode-task approach (feat: automate proxy setup for local testing #7391/feat: vscode task config for launching dev proxy #7799) as the batteries-included, editor-agnostic flow.
• Docs: new "Local asset preview" section under "Testing your branch".

How did you test this change?

Limited local testing: npm run dev opens the browser with local assets, and editing .scss/.js auto-reloads the page. Not extensively battle-tested yet.