Web: visual design pass (issue #26)

Addresses the screenshot-review feedback in #26. No framework or build
step added — all of this stays within the existing plain HTML/CSS/
vanilla-JS + go:embed architecture.

- Nav: re-enable the bottom tab bar that was already built and
  switched off (Queue/On-call/Alerts/Team + a "More" sheet for
  Stats/Admin/Account), replacing the hamburger on phone width.
- Queue: chip counts, a scroll fade on the filter row, a
  "Triggered Xh ago" + severity label per row, a chevron on the team
  switcher so it reads as a dropdown.
- On-call: collapse repeated same-person days into shift bars (week
  view and "your shifts" both), show the week as a date range with
  the ISO week number as secondary text, split "Current shift" out
  from "Next shifts" with "ends in Nd", a pill badge + row highlight
  for "you".
- Incident detail: fix the actual bug behind the duplicate
  "acknowledged" timeline entries (acknowledgeIncident's UPDATE had no
  guard on the incident's current status, so acknowledging an
  already-acknowledged incident silently re-logged the event — now
  idempotent, with regression tests on both the authenticated route
  and the ntfy ack-button route). Relabel escalation re-pages so they
  don't look like the same page landing twice. Copy the primary action
  up near the top. Label the "···" button. Group the timeline by
  phase (triggered/acknowledged/resolved). Add an "at a glance"
  summary row (duration/severity/responsible) and collapse the group
  labels by default.
- Team overview: reword the vague copy ("One owner." etc.) into plain
  labels.
- Empty states: fill in missing icons/one-liners across queue,
  alerts, stats and the incident timeline.
- CSS: fix card padding bugs, verify link contrast already passes AA,
  introduce a --fs-* type-scale token set and migrate the few
  genuinely isolated cases onto it (left sizes tied to a fixed shape,
  a deliberately prominent display, or a non-negotiable constraint
  like the iOS-zoom-prevention input size as documented exceptions
  rather than guess at a render this change can't see).

Verified with the full fmt/lint/test/helm-lint gate, plus a live
instance against the test DB with seeded incidents and schedule data
to trace the on-call grouping and timeline phase-splitting logic
against real API responses.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/web/static/js/stats.js

@@ -80,7 +80,7 @@ function render() {
   if (error && !data) body = h('div', { class: 'load-error', text: error });
   else if (!data) body = spinner();
   else if (!data.incidents.total && !data.byHour.some((x) => x.count)) {
-    body = emptyState('No data in this range', '', 'chart');
+    body = emptyState('No data in this range', 'Nothing happened in this window.', 'chart');
   } else {
     body = h('div', { class: 'stats' },
       error && h('div', { class: 'load-error', text: `Showing older data: ${error}` }),