Disable SSR for the renderer app shell

[Noshkas]

Fixes #515

Summary

• Move the Electron-dependent app wrapper out of _app into a client-only app shell loaded with next/dynamic({ssr: false}).
• Remove the mounted-state workaround from _app, so Next handles skipping server rendering directly.
• Scope the custom electron-renderer webpack target to the client build only; the server/export build keeps the normal Node target and can evaluate dynamic imports without requiring window.
• Update CircleCI to a currently supported macOS image while keeping Kap on Node 16 for the existing dependency graph.

Verification

• npx -p node@16 -c 'node ./node_modules/.bin/xo' passed with existing warnings only.
• npx -p node@16 -c 'node ./node_modules/typescript/bin/tsc --noEmit -p tsconfig.json'
• npx tsc --noEmit -p renderer/tsconfig.json
• npx -p node@16 -c 'node ./node_modules/next/dist/bin/next build renderer && node ./node_modules/next/dist/bin/next export renderer'
• Checked exported renderer HTML, e.g. renderer/out/editor.html, renders an empty #__next shell rather than server-rendering the Electron UI.
• CircleCI build passed on this PR after the CI image update.

Note: I installed dependencies locally with yarn install --frozen-lockfile --ignore-scripts --ignore-engines because this older lockfile rejects modern Node during the engine check, then ran node node_modules/electron/install.js so the renderer build could import Electron during page-data collection.