Serve the Progmasoft mark as the site iconfeature/site-icons

The sites declared no icon and /favicon.ico answered 404, so search
engines and assistants that show a site's icon (DuckDuckGo, ChatGPT)
fell back to a gray placeholder; the only copy of the mark was the BIMI
logo for mail.

The app now has the Next.js icon files, so every page links them:

- app/icon.svg: the four squares of the BIMI logo, cropped to the mark
  so it fills a browser tab, on a transparent field.
- app/favicon.ico: the same mark at 16, 32 and 48 pixels (PNG entries),
  for clients that ask for /favicon.ico without reading the page.
- app/apple-icon.png: 180 pixels on the white field of the BIMI logo,
  since iOS fills transparency with black.

The proxy rewrites paths per host, which would send /icon.svg on the
account, ViGet and the other hosts into their own page trees; the
matcher now skips the two new files as it already skipped favicon.ico,
so every Progmasoft host serves the same icons.

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