feat!: stats as a section instead of an overlay

Stats was the one full-screen view reached by a key of its own rather
than by tab, and the interface was less coherent for it. It is now a
section sitting third, after Alerts, and behaves like every other one:
tab in, tab out, r to refresh.

Three things fall out of the move. It auto-refreshes for the first time
— the tick handler skips every non-dashboard mode, which is why the
overlay never updated while it was open. Its error path no longer forces
the queue back into view on a failed fetch, an assumption that only made
sense while stats floated above the dashboard. And first-visit loading
keys off a statsLoaded flag rather than slice emptiness, because the
three empty slices a quiet server returns are a real answer, not a
missing one; the loading placeholder is likewise suppressed once
something has been drawn, so a background refresh cannot blank the page
out from under whoever is reading it.

The S key is gone, and with it the ability to peek at statistics from an
open incident and land back on it. That round-trip was the only thing
statsReturnMode bought, and it was the whole reason stats needed a mode.

internal/tui/view_test.go

@@ -6,6 +6,7 @@ import (
 	"testing"
 	"time"
 
+	tea "github.com/charmbracelet/bubbletea"
 	"github.com/yeniklas/terdut-tui/internal/api"
 )
 
@@ -239,7 +240,7 @@ func TestView_TabsAndDashboardRender(t *testing.T) {
 	m.rebuildIncidentTable()
 
 	mustContain(t, m.View(),
-		"Incidents", "Alerts", "Archived", "Schedule", "Users",
+		"Incidents", "Alerts", "Stats", "Archived", "Schedule", "Users",
 		"Triggered: 1", "filter: open",
 		"DiskFull", "critical", "admin",
 		"enter·detail")
@@ -254,6 +255,27 @@ func TestView_EmptyStates(t *testing.T) {
 	mustContain(t, m.View(), "No archived incidents.")
 }
 
+// The stats page renders inside the normal section chrome now, so it has to
+// survive the real path: a window size message sizes the viewport and fills it.
+func TestView_StatsSectionRendersInPlace(t *testing.T) {
+	m := NewModel(nil, "http://test", time.Minute)
+	m.connected = true
+	m.incidentStats = &api.IncidentStats{Total: 3, Triggered: 1}
+	m.topAlerts = []api.TopAlert{{Name: "DiskFull", Count: 4}}
+	m.statsLoaded = true
+
+	next, _ := m.Update(tea.WindowSizeMsg{Width: 120, Height: 40})
+	m = next.(Model)
+	m.activeSection = sectionStats
+
+	out := m.View()
+	mustContain(t, out, "Stats", "Incident Response", "Top Alerts", "DiskFull",
+		"tab·section")
+	if strings.Contains(plain(out), "Loading statistics") {
+		t.Error("loaded stats should not show the loading placeholder")
+	}
+}
+
 func TestView_ConnectionError(t *testing.T) {
 	m := sized()
 	m.connected = false