feat: ship a web app manifest and home-screen icons

Adding the app to a phone home screen produced a blank tile: index.html
linked only an SVG favicon, and Safari ignores SVG for apple-touch-icon,
substituting a screenshot of the page. There was no manifest at all, so
Android had nothing to install against either.

Generate the rasters from public/logo.svg with sharp, which the repo
already carries. The output is committed so a plain `vite build` needs no
image toolchain; re-run scripts/generate-icons.mjs after changing the
logo. The logo's viewBox is wider than it is tall, so each icon centres
the mark on a square #0a0a0f canvas rather than stretching it, inset to
78% normally and 54% for the maskable variant to survive Android's
circular crop.

The SVG favicon stays as the browser tab icon.

scripts/format-scope.json

@@ -981,5 +981,7 @@
   "src/tools/globePanel.test.mjs",
   "src/costEndpointThrottle.test.mjs",
   "src/sources/gbfsStations.test.mjs",
-  "scripts/qa-gbfs-names.mjs"
+  "scripts/qa-gbfs-names.mjs",
+  "scripts/generate-icons.mjs",
+  "src/tooling/webAppIcons.test.mjs"
 ]