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/api/incident_store.go

@@ -261,14 +261,17 @@ func resolveIfSettled(ctx context.Context, q querier, incidentID int64) (bool, e
 }
 
 // acknowledgeIncident records that userID has picked an incident up, and reports
-// whether it changed anything — an already-resolved incident is left alone.
-// Shared by the authenticated handler and the Acknowledge button in a push
-// notification, so both write the same state and the same timeline entry.
+// whether it changed anything — an already-resolved or already-acknowledged
+// incident is left alone, so a second acknowledge (a retried request, or a
+// stale push notification tapped after the web UI already acked it) is a
+// no-op rather than a second "acknowledged" timeline entry. Shared by the
+// authenticated handler and the Acknowledge button in a push notification,
+// so both write the same state and the same timeline entry.
 func acknowledgeIncident(ctx context.Context, q querier, incidentID, userID int64) (bool, error) {
 	res, err := q.ExecContext(ctx, `
 		UPDATE incidents
 		   SET status = 'acknowledged', acknowledged_by = $1, acknowledged_at = $2
-		 WHERE id = $3 AND resolved_at IS NULL`,
+		 WHERE id = $3 AND status = 'triggered'`,
 		userID, time.Now().Unix(), incidentID)
 	if err != nil {
 		return false, err