Keep the notifications worth interrupting for

Splitting system messages out of the modal was right for payout receipts,
but it would have silenced two that a member genuinely loses money by
ignoring:

  - "@someone is trying to buy. Link your wallet so it pays you" — a sale
    is blocked right now, and the referral is lost permanently once it
    routes to someone else.
  - "You missed 43 POL on InstantAdPay" — a payout passed them by, and the
    message explains exactly how to stop the next one doing the same.

So the dividing line is not system-versus-human, it is "does this need you
to do something". Those two become kind 'alert' and still interrupt; the
receipts stay kind 'notice' and stay in the inbox.

The modal no longer credits an alert to a person either. It was saying "A
message from @martbost" over machine-generated text, because system mail
is sent by member 1 at ADMIN_EMAIL. Alerts now read "Action needed on your
account".

Also reclassified "X is now in your line for good" as a notice — it is
good news about a referral the member gained, with nothing at stake.

qa/messages-notice.mjs now covers both lanes: 14 checks, including that an
alert interrupts and a flood of 25 receipts does not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

public/assets/my.js

@@ -1302,7 +1302,11 @@
     if (!$('msgModal') || !msg || msgModalShown.has(msg.id)) return;
     if (!$('msgModal').hidden) return;               // one already open
     msgModalShown.add(msg.id);
-    $('mmFrom').textContent = 'A message from ' + (msg.fromName || 'your sponsor');
+    // An alert is generated by the system, so crediting it to a person ("A message from
+    // @martbost") is both wrong and misleading about what the member is looking at.
+    $('mmFrom').textContent = msg.kind === 'alert'
+      ? 'Action needed on your account'
+      : 'A message from ' + (msg.fromName || 'your sponsor');
     $('mmSubject').textContent = msg.subject || '';
     $('mmBody').innerHTML = msg.body || ''; // server-sanitized
     $('msgModal').hidden = false;