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.

public/sw.js

@@ -9,7 +9,7 @@
  *
  * Bumping CACHE_VERSION is how a new deploy invalidates old assets.
  */
-const CACHE_VERSION = "freezmo-v2";
+const CACHE_VERSION = "freezmo-v3";
 const SHELL_CACHE = `${CACHE_VERSION}-shell`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;