Brand: inbuxa's own kitten replaces ihasmail's cat

The webmail showed ihasmail's cat-and-envelope as inbuxa's mark. The new
mark keeps the family's face, paws and colors, over a server with a bay
for each piece of the suite: the letter (webmail), a prompt (console),
status lights (server).

- img/inbuxa-mark.png (header, sign-in, About) and img/logo.png (the
  mail empty state and the custom-name fallback).
- favicon.ico, favicon-64, apple-touch-icon (opaque white, as before),
  icon-192/512, and icon-maskable, now on an opaque ground with the
  mark inside the safe circle.
- Login.tsx: 120x126, the new mark's proportions; 120x143 would have
  stretched it. Every other use sizes by one dimension.
- The service worker fetches images network-first, so installed copies
  pick the new ones up without a cache version bump.

web/src/views/Login.tsx

@@ -92,7 +92,7 @@ export function LoginPage() {
     <div className="login-page">
       <form className="login-card" onSubmit={submit}>
         <div className="logo">
-          <img src={withBase(appName === DEFAULT_APP_NAME ? "/img/inbuxa-mark.png" : "/img/logo.png")} alt="" width={120} height={143} />
+          <img src={withBase(appName === DEFAULT_APP_NAME ? "/img/inbuxa-mark.png" : "/img/logo.png")} alt="" width={120} height={126} />
           {/* A product name, not a word: not translated, and not guessed at
               from the page it is on. INBUXA's is its wordmark. */}
           <h1 className="notranslate" translate="no">