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/api/router.go

@@ -62,6 +62,10 @@ func NewRouter(db *sql.DB, notify NotifyConfig, cfg config.Config) http.Handler
 		r.Use(AuthMiddleware(db))
 
 		r.Get("/api/me", handleMe(db))
+		r.Put("/api/me/onboarding", handleDismissOnboarding(db))
+		// Proves the topic works, which is the only part of "notifications are
+		// set up" that the person holding the phone can confirm.
+		r.Post("/api/me/notify/test", handleTestNotification(notify, db))
 
 		// Readable by anyone signed in: the queue's assignment control and the
 		// on-call schedule both need to name people.