Checks send no Referer header, so Hetzner answers (closes #191)

The Hetzner speed-test servers close the connection without an answer
when the request has a Referer header. The browser sends one unless the
page comes with the image's Referrer-Policy: no-referrer header, which a
static host and make dev do not send. Each check now sets
referrerPolicy: "no-referrer".

Model: opus-5-5

README.md

@@ -413,10 +413,12 @@ Local hosts are tracked separately from WAN stats.
 
 ### Latency measurement
 
-GET requests to each target's URL as written, with `mode: 'no-cors'` and
-`cache: 'no-store'`, timed with `performance.now()`. No query string is added:
-the Hetzner speed-test servers close the connection without an answer when the
-URL has one, and `no-store` keeps the browser's cache out of the measurement.
+GET requests to each target's URL as written, with `mode: 'no-cors'`,
+`cache: 'no-store'` and `referrerPolicy: 'no-referrer'`, timed with
+`performance.now()`. The Hetzner speed-test servers close the connection without
+an answer when the URL has a query string or the request has a `Referer` header.
+So no query string is added, and no `Referer` header is sent, wherever the page
+is served from. `no-store` keeps the browser's cache out of the measurement.
 Each check times out after 80% of the refresh interval (24 seconds at 30
 seconds) and is then recorded as a timeout, so a round's checks have all
 finished before the next round is due. When no WAN host answers, a recovery