web: generate favicon before npm run dev too

favicon.ico is gitignored and produced by prebuild only, so `npm run dev`
served a 404 for it. Mirror apps/desktop, whose dev script already runs
the icon generator first.

web/package.json

@@ -4,6 +4,7 @@
   "version": "0.0.0",
   "type": "module",
   "scripts": {
+    "predev": "node ../scripts/generate-icons.mjs --source .. --out ..",
     "dev": "vite",
     "prebuild": "node ../scripts/generate-icons.mjs --source .. --out ..",
     "build": "node ../scripts/build/web.mjs",