#785: [retry] #47 Stage 1: move struct App + ShellApp impl out of src/gtk/ into a backend-neutral src/app.rs - #791

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/cache@v4, actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/