[Improvement] Add manifest to make it "installable"

[opoto]

Adding a manifest file would to make the app installable. The main benefit is that it is then launched into a decided window with no browser decoration (URL bar, navigation buttons, tabs...), which is particularly useful on mobile where each pixel counts!

Two steps needed:

1. Create a manifest.json file, such as

{
  "name": "gpx.studio",
  "short_name": "gpx.studio",
  "start_url": ".",
  "display": "standalone",
  "background_color": "#fff",
  "description": "The online GPX file editor",
  "icons": [
    {
      "src": "icon.svg",
      "sizes": "any"
    }
  ]
}

2. Declare the manifest in index.html

<link rel="manifest" href="manifest.json" />

[vcoppe]

Could use https://github.com/vite-pwa/vite-plugin-pwa