feat(pwa): the PNG icons the manifest and iOS have always wanted

The manifest pointed at icon.svg and nothing else. Chrome and Firefox
install happily on that; Safari does not read the manifest's icons at all
and looks for /apple-touch-icon.png by name, so an installed PWA on iOS got
a screenshot of the page as its home-screen icon.

This was written down as blocked: there is no SVG renderer on this machine,
which is why the references were removed rather than left pointing at files
that did not exist. Playwright is an SVG renderer, and it has been a
dependency the whole time.

scripts/generate-icons.js renders the same SVG at 192, 512 and 180. A script
rather than a test, so CI never runs it and never rewrites a tracked binary
— run it when the icon changes. The smoke test already fetches every icon
the manifest names, and now asks for the Apple one by name too, so a
reference with no file behind it fails there rather than on somebody's
phone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/static/manifest.webmanifest

@@ -12,7 +12,25 @@
       "src": "/icon.svg",
       "sizes": "any",
       "type": "image/svg+xml",
-      "purpose": "any maskable"
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
     }
   ]
 }