Use the new app icon everywhere

Source artwork committed as assets/app-icon-source.jpeg (1024x1024 navy
rounded square) and scripts/make-icons.mjs rewritten to render every icon
from it with headless Chromium:

- The master is masked at full resolution first (a rounded rect 4px inside
  the artwork's own radius), so the JPEG's anti-aliased white edge never
  survives and small sizes downscale with clean alpha.
- PWA icons (192/512) and favicon-32 keep the rounded square with
  transparent corners; apple-touch 180/167/152 are full-bleed (iOS paints
  transparency black) with the artwork zoomed 20% to crop its corners;
  maskable-512 keeps the artwork inside the 80% safe circle on a matching
  navy field.
- iOS splash screens regenerated with the new mark.
- app/favicon.ico regenerated (PNG payload in an ICO container); the old
  app/icon.svg removed so the browser tab can't fall back to it.
- Install sheet's app card now shows the real icon, so the home-screen
  icon is recognisable.
- Service worker CACHE_VERSION bumped to freezmo-v3 so installed clients
  pick up the new assets.
- README updated (source artwork + script description).

Verified: pwa 16/16, install sheet 64/64, final ux 12/12, key gate 49/49,
zoom 6/6; every icon route serves 200 and the manifest/head reference the
new files.

app/icon.svg

@@ -1,14 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" width="64" height="64">
-  <defs>
-    <linearGradient id="frost" x1="0" y1="0" x2="1" y2="1">
-      <stop offset="0%" stop-color="#38bdf8" />
-      <stop offset="100%" stop-color="#0369a1" />
-    </linearGradient>
-  </defs>
-  <rect width="64" height="64" rx="16" fill="url(#frost)" />
-  <g stroke="#ffffff" stroke-width="3.4" stroke-linecap="round" fill="none">
-    <path d="M32 12v40M14.7 22l34.6 20M49.3 22 14.7 42" />
-    <path d="M32 20.5 26.8 15.3M32 20.5l5.2-5.2M32 43.5l-5.2 5.2M32 43.5l5.2 5.2" />
-    <path d="M20.7 27.5l-6.6-1.7M20.7 36.5l-6.6 1.7M43.3 27.5l6.6-1.7M43.3 36.5l6.6 1.7" />
-  </g>
-</svg>