feat(tui): add state change history view with outage duration

Full-screen scrollable history view accessible via [h] from detail
panel. Shows all state transitions with computed outage durations,
event density sparkline for flapping detection, and summary stats.

- Detail panel STATE CHANGES now shows outage duration per recovery
- Event density sparkline highlights flapping periods
- Summary footer: event count, outage count, avg outage duration
- Vim-style navigation (j/k/g/G) + mouse scroll in history view

internal/tui/view_history.go

@@ -0,0 +1,188 @@
+package tui
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
+	"github.com/charmbracelet/lipgloss"
+)
+
+type historyStats struct {
+	totalEvents   int
+	outageCount   int
+	totalDowntime time.Duration
+}
+
+func computeOutageDuration(changes []models.StateChange, idx int) time.Duration {
+	sc := changes[idx]
+	if sc.ToStatus != "UP" {
+		return 0
+	}
+	if idx+1 >= len(changes) {
+		return 0
+	}
+	prev := changes[idx+1]
+	if prev.ToStatus == "UP" {
+		return 0
+	}
+	dur := sc.ChangedAt.Sub(prev.ChangedAt)
+	if dur < 0 {
+		return 0
+	}
+	return dur
+}
+
+func computeHistoryStats(changes []models.StateChange) historyStats {
+	var s historyStats
+	s.totalEvents = len(changes)
+	for i := range changes {
+		dur := computeOutageDuration(changes, i)
+		if dur > 0 {
+			s.outageCount++
+			s.totalDowntime += dur
+		}
+	}
+	return s
+}
+
+func stateChangeSparkline(changes []models.StateChange, width int) string {
+	if len(changes) < 2 || width < 4 {
+		return ""
+	}
+
+	oldest := changes[len(changes)-1].ChangedAt
+	newest := changes[0].ChangedAt
+	span := newest.Sub(oldest)
+	if span <= 0 {
+		return ""
+	}
+
+	buckets := make([]int, width)
+	for _, sc := range changes {
+		pos := int(float64(sc.ChangedAt.Sub(oldest)) / float64(span) * float64(width-1))
+		if pos >= width {
+			pos = width - 1
+		}
+		if pos < 0 {
+			pos = 0
+		}
+		buckets[pos]++
+	}
+
+	maxVal := 0
+	for _, v := range buckets {
+		if v > maxVal {
+			maxVal = v
+		}
+	}
+	if maxVal == 0 {
+		return ""
+	}
+
+	var sb strings.Builder
+	for _, v := range buckets {
+		if v == 0 {
+			sb.WriteRune('·')
+			continue
+		}
+		idx := int(float64(v) / float64(maxVal) * 7)
+		if idx > 7 {
+			idx = 7
+		}
+		ch := string(sparkChars[idx])
+		switch {
+		case v >= 3:
+			sb.WriteString(dangerStyle.Render(ch))
+		case v >= 2:
+			sb.WriteString(warnStyle.Render(ch))
+		default:
+			sb.WriteString(subtleStyle.Render(ch))
+		}
+	}
+	return sb.String()
+}
+
+func (m Model) buildHistoryContent() string {
+	var b strings.Builder
+
+	reasonWidth := m.termWidth - chromePadH - 55
+	if reasonWidth < 10 {
+		reasonWidth = 10
+	}
+	if reasonWidth > 60 {
+		reasonWidth = 60
+	}
+
+	for i, sc := range m.historyChanges {
+		ts := sc.ChangedAt.Format("2006-01-02 15:04")
+
+		arrow := subtleStyle.Render(sc.FromStatus) + " → "
+		if sc.ToStatus == "UP" {
+			arrow += specialStyle.Render(sc.ToStatus)
+		} else {
+			arrow += dangerStyle.Render(sc.ToStatus)
+		}
+
+		durStr := ""
+		if dur := computeOutageDuration(m.historyChanges, i); dur > 0 {
+			durStr = warnStyle.Render("outage " + fmtDuration(dur))
+		}
+
+		reason := ""
+		if sc.ErrorReason != "" && sc.ToStatus != "UP" {
+			reason = dangerStyle.Render(limitStr(sc.ErrorReason, reasonWidth))
+		}
+
+		fmt.Fprintf(&b, "  %-18s %s  %-12s %s\n", ts, arrow, durStr, reason)
+	}
+
+	return b.String()
+}
+
+func (m Model) viewHistoryPanel() string {
+	var b strings.Builder
+
+	header := "  " + titleStyle.Render("STATE HISTORY: "+m.historySiteName)
+	header += "  " + subtleStyle.Render("[q] Back")
+	b.WriteString(header + "\n")
+
+	divWidth := m.termWidth - chromePadH - 4
+	if divWidth < 40 {
+		divWidth = 40
+	}
+	b.WriteString("  " + subtleStyle.Render(strings.Repeat("─", divWidth)) + "\n")
+
+	sparkline := stateChangeSparkline(m.historyChanges, divWidth)
+	if sparkline != "" {
+		b.WriteString("  " + sparkline + "\n")
+		b.WriteString("  " + subtleStyle.Render(strings.Repeat("─", divWidth)) + "\n")
+	}
+
+	b.WriteString(fmt.Sprintf("  %-18s %-17s %-12s %s\n",
+		subtleStyle.Render("TIME"),
+		subtleStyle.Render("TRANSITION"),
+		subtleStyle.Render("DURATION"),
+		subtleStyle.Render("REASON")))
+
+	if len(m.historyChanges) == 0 {
+		b.WriteString("\n  " + subtleStyle.Render("No state changes recorded") + "\n")
+	} else {
+		b.WriteString(m.historyViewport.View())
+	}
+
+	b.WriteString("\n  " + subtleStyle.Render(strings.Repeat("─", divWidth)) + "\n")
+
+	stats := computeHistoryStats(m.historyChanges)
+	parts := []string{fmt.Sprintf("%d events", stats.totalEvents)}
+	if stats.outageCount > 0 {
+		parts = append(parts, fmt.Sprintf("%d outages", stats.outageCount))
+		avg := stats.totalDowntime / time.Duration(stats.outageCount)
+		parts = append(parts, "avg outage "+fmtDuration(avg))
+	}
+	b.WriteString("  " + subtleStyle.Render(strings.Join(parts, " │ ")) + "\n")
+	b.WriteString("  " + subtleStyle.Render("[j/k/↑/↓] Scroll  [q/Esc] Back"))
+
+	return lipgloss.NewStyle().Padding(1, 2).Render(b.String())
+}