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/js/queue.js

@@ -4,6 +4,7 @@ import * as api from './api.js';
 import { h, clear, badge, emptyState, spinner } from './ui.js';
 import { age, until, isFuture, severityClass, labelSummary } from './format.js';
 import { state, myID } from './state.js';
+import * as onboarding from './onboarding.js';
 import { navigate } from './app.js';
 
 // The same filters as the TUI's `f` cycle, plus archived ones to get back to.
@@ -25,6 +26,8 @@ const EMPTY = {
   archived: ['Nothing archived', ''],
 };
 
+onboarding.onRerender(() => renderList());
+
 let filter = loadFilter();
 let teamFilter = loadTeamFilter(); // '' for every team the viewer is in
 let items = null; // null while loading
@@ -89,6 +92,7 @@ export async function refresh({ fresh = false } = {}) {
     const query = teamFilter ? { ...f.query, team_id: teamFilter } : f.query;
     const cached = filter === 'open' && !fresh && !teamFilter;
     const result = cached ? state.open : await api.incidents(query);
+    await onboarding.load();
     if (requested !== filter) return;
     items = result;
     error = null;
@@ -153,20 +157,22 @@ function renderChips() {
 
 function renderList() {
   const el = document.getElementById('queue-list');
+  const checklist = onboarding.card();
   if (error && !items) {
-    clear(el, h('div', { class: 'load-error', text: error }));
+    clear(el, checklist, h('div', { class: 'load-error', text: error }));
     return;
   }
   if (!items) {
-    clear(el, spinner());
+    clear(el, checklist, spinner());
     return;
   }
   if (!items.length) {
     const [title, text] = EMPTY[filter];
-    clear(el, emptyState(title, text, filter === 'open' ? 'checkCircle' : null));
+    clear(el, checklist, emptyState(title, text, filter === 'open' ? 'checkCircle' : null));
     return;
   }
   clear(el,
+    checklist,
     error && h('div', { class: 'load-error', text: `Showing older data: ${error}` }),
     items.map((inc, i) => row(inc, i)),
   );