Add the sign-up page and the first-run checklist

Second half of #7. The API could create accounts from invite links since
the last change; this is the part somebody can actually use.

/signup is the one route that works without a session. It asks the server
what it may offer before showing anything: an invite link that is good
names the team it leads to, a link that is not says so before somebody
picks a password rather than after, and an invite-only server with no
link says that instead of presenting a form it will refuse. The login
card only offers "create one" when sign-up is open, so the door nobody
can walk through is not advertised.

Signing up signs you in and lands on the queue, because the alternative
is a form saying "now go and log in" about the credential just chosen.

The checklist is the other half. Four things have to be true before an
alert reaches a phone -- a notification topic, somebody on the rota, an
alert source, and an alert that has actually arrived -- and on a fresh
install none of them are. It sits above the queue until they are.

It is computed from the data rather than from stored progress: a topic is
set or it is not, an integration exists or it does not. That means it
cannot claim a step is done when it is not, and it comes back by itself
if somebody deletes their integration a month later. The only stored
state is the dismissal, which is per user and not per browser --
finishing on a laptop should not leave the phone nagging.

The topic step is the only one the checklist can finish itself, and the
only proof that counts is a phone buzzing, so there is a test push.
POST /api/me/notify/test publishes directly rather than through the
outbox, which requires an incident this deliberately does not have. Its
failure is the useful part: a wrong topic, a rejected token and an ntfy
that is down all look identical from the phone, which is silence, so the
error comes back to the browser instead.

Verified against a live server with a real ntfy stand-in, the whole path:
an owner mints an invite, the sign-up page reports it valid and names the
team, the invitee signs up and is signed in as a member of that team, the
checklist's four questions answer correctly on a fresh install, a test
push is refused with no topic and delivered with one -- "PAGED
terdut-owner | terdut test" -- and the dismissal survives a reload.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

internal/web/static/app.css

@@ -687,3 +687,23 @@ kbd {
   border-radius: 6px; padding: 8px; font-size: 12px;
 }
 .key-url code { word-break: break-all; }
+
+/* --- onboarding checklist ------------------------------------------------
+   Sits above the queue until it is finished or hidden. Deliberately plain:
+   it is a list of things to do, not a celebration. */
+.onboarding { border-left: 3px solid var(--accent); }
+.onboarding-head { display: flex; align-items: center; gap: 10px; }
+.onboarding-head h2 { flex: 1; margin: 0; }
+.checklist { list-style: none; margin: 12px 0 0; padding: 0; display: flex; flex-direction: column; gap: 12px; }
+.checklist .step { display: flex; gap: 10px; align-items: flex-start; }
+.checklist .step p { margin: 2px 0 0; }
+.step-mark {
+  flex: none; width: 20px; height: 20px; border-radius: 50%;
+  border: 1px solid var(--border-strong); color: var(--accent);
+  display: flex; align-items: center; justify-content: center; font-size: 13px;
+}
+.step.done .step-mark { border-color: var(--accent); }
+.step.done > div > strong { color: var(--muted); text-decoration: line-through; }
+.step-actions { display: flex; gap: 6px; margin-top: 6px; flex-wrap: wrap; }
+
+.signup-intro { margin: 0 0 4px; font-size: 14px; color: var(--muted); }