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.

components/InstallSheet.tsx

@@ -12,7 +12,6 @@ import {
   EllipsisIcon,
   PlusIcon,
   ShareIcon,
-  SnowflakeIcon,
   XIcon,
 } from "@/components/icons";
 import { Spinner } from "@/components/ui";
@@ -269,12 +268,13 @@ export function InstallSheet() {
 
                 {/* app card */}
                 <div className="mt-1 flex items-center gap-3 rounded-2xl border border-[var(--install-border)] bg-[var(--install-card)] p-3">
-                  <span
+                  {/* eslint-disable-next-line @next/next/no-img-element -- static app icon, no optimisation needed */}
+                  <img
+                    src="/icons/icon-192.png"
+                    alt=""
                     aria-hidden
-                    className="grid size-11 shrink-0 place-items-center rounded-xl bg-[var(--text)] text-[var(--page)] dark:border dark:border-[var(--install-border)] dark:bg-[var(--install-chip)] dark:text-[var(--text)]"
-                  >
-                    <SnowflakeIcon className="size-5" />
-                  </span>
+                    className="size-11 shrink-0 rounded-[11px]"
+                  />
                   <span className="min-w-0">
                     <span className="block text-[15px] font-semibold leading-tight text-[var(--text)]">
                       Freezmo