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/ui/InbuxaWordmark.tsx

@@ -1,9 +1,9 @@
 /**
  * The INBUXA wordmark, "inbuxa" in Space Grotesk Bold, as vector paths in the
  * current text color, so it reads on every palette, light or dark. The mark
- * beside it is `/img/inbuxa-mark.png`: ihasmail's own cat and envelope, which
- * INBUXA's logo reuses unchanged. The paths are the same ones INBUXA Admin
- * draws, from the INBUXA logo bundle.
+ * beside it is `/img/inbuxa-mark.png`: inbuxa's own kitten, the family's cat
+ * over a server with a bay for each piece of the suite. The paths are the
+ * same ones inbuxa Admin draws, from the inbuxa logo bundle.
  *
  * ihasmail-inbuxa only. Public ihasmail keeps its own name.
  */