harden: take a request's origin from the frame that sent it (closes #407)

Where the browser gives no sender.origin (Firefox before 126), the
background credited a page's request to the tab's page, so a frame from
another site counted as the site embedding it, and with no tab it used
an origin the page wrote into the message. It now uses the origin of
sender.url, the frame that sent the message, and refuses the request
with code 4100 when the browser gives neither. The content script no
longer writes an origin into the message.

Model: opus-5-5

src/content/index.js

@@ -30,7 +30,6 @@ window.addEventListener("message", (event) => {
         id,
         method,
         params,
-        origin: location.origin,
     })
         .then((response) => {
             if (response) {