fix: trim the body before sealing instead of appending a newline

The body was right-trimmed and then given a trailing newline back, which is
reasonable for a mail body and fatal for a published twtxt record: smolboid
refuses a post whose text carries a line break, because a record is one line
of a feed.

Trimming belongs here, before the seal, so the signature covers exactly the
text the author meant. A receiver cannot do it — stripping is not invertible,
so it could no longer rebuild the signed bytes to verify them.

web/js/app.js

@@ -950,8 +950,14 @@ async function sendMail(toText, subject, body, replyTo, anonymous) {
   // first reply from them reaches our main tier.
   const accepted = store.accepted(addr.short);
   if (accepted?.active) fields.push(["Accept", b32encode(tokenFor(master, accepted.identity))]);
+  // Trimmed before the payload is signed, so the signature covers exactly the
+  // text the author meant. A trailing newline is not content — but it becomes
+  // content the moment it is signed, and nothing downstream can strip it
+  // without invalidating the signature. smolboid, for one, refuses a post
+  // whose text carries a line break, because a published record is one line
+  // of a feed.
   const bodyBytes = utf8Bytes(buildFrontmatter(fields.filter(([, v]) => v),
-    body.replace(/\s+$/, "") + "\n"));
+    body.trim()));
   const envelope = seal(me, recipient, bodyBytes);
   // §5.8: our token for their mailbox, if they have given us one.
   const held = store.tokenFrom(addr.short);