refactor(tui): decompose god files into single-concern modules

tui.go (1032→164) and tab_sites.go (993→482) violated "small functions"
and "testable in isolation" standards. Extracted 6 new files by concern:

- format.go: pure formatting functions (fmtLatency, fmtUptime, etc.)
- sparkline.go: sparkline rendering (latency, heartbeat, group)
- update.go: Update method decomposed into 15 named handlers
- view_dashboard.go: View, dashboard composition, tab bar, footer
- view_detail.go: site detail panel
- data.go: data refresh with extracted sortSitesForDisplay/filterSites

Added 17 unit tests for the newly-testable pure functions covering
format, sparkline, sort ordering, and filter logic. No behavioral
changes — strict move-and-extract refactor.

internal/tui/sparkline.go

@@ -0,0 +1,174 @@
+package tui
+
+import (
+	"strings"
+	"time"
+)
+
+var sparkChars = []rune{'▁', '▂', '▃', '▄', '▅', '▆', '▇', '█'}
+
+func latencySparkline(latencies []time.Duration, statuses []bool, width int) string {
+	if len(latencies) == 0 {
+		return subtleStyle.Render(strings.Repeat("·", width))
+	}
+
+	samples := latencies
+	sampledStatuses := statuses
+	if len(samples) > width {
+		samples = samples[len(samples)-width:]
+		if len(sampledStatuses) > width {
+			sampledStatuses = sampledStatuses[len(sampledStatuses)-width:]
+		}
+	}
+
+	minL, maxL := samples[0], samples[0]
+	for _, l := range samples {
+		if l < minL {
+			minL = l
+		}
+		if l > maxL {
+			maxL = l
+		}
+	}
+
+	var sb strings.Builder
+	if remaining := width - len(samples); remaining > 0 {
+		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
+	}
+	spread := maxL - minL
+	for i, l := range samples {
+		idx := 0
+		if spread > 0 {
+			idx = int(float64(l-minL) / float64(spread) * 7)
+			if idx > 7 {
+				idx = 7
+			}
+		}
+		ch := string(sparkChars[idx])
+		isDown := i < len(sampledStatuses) && !sampledStatuses[i]
+		if isDown {
+			sb.WriteString(dangerStyle.Render(ch))
+		} else {
+			ms := l.Milliseconds()
+			if ms < 200 {
+				sb.WriteString(specialStyle.Render(ch))
+			} else if ms < 500 {
+				sb.WriteString(warnStyle.Render(ch))
+			} else {
+				sb.WriteString(dangerStyle.Render(ch))
+			}
+		}
+	}
+	return sb.String()
+}
+
+func heartbeatSparkline(statuses []bool, width int) string {
+	if len(statuses) == 0 {
+		return subtleStyle.Render(strings.Repeat("·", width))
+	}
+
+	samples := statuses
+	if len(samples) > width {
+		samples = samples[len(samples)-width:]
+	}
+
+	var sb strings.Builder
+	if remaining := width - len(samples); remaining > 0 {
+		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
+	}
+	for _, up := range samples {
+		if up {
+			sb.WriteString(specialStyle.Render("▁"))
+		} else {
+			sb.WriteString(dangerStyle.Render("█"))
+		}
+	}
+	return sb.String()
+}
+
+func (m Model) groupSparkline(groupID int, width int) string {
+	allSites := m.engine.GetAllSites()
+	var childStatuses [][]bool
+	for _, s := range allSites {
+		if s.ParentID == groupID && !s.Paused && !m.isMonitorInMaintenance(s.ID) {
+			hist, _ := m.engine.GetHistory(s.ID)
+			if len(hist.Statuses) > 0 {
+				childStatuses = append(childStatuses, hist.Statuses)
+			}
+		}
+	}
+
+	if len(childStatuses) == 0 {
+		return subtleStyle.Render(strings.Repeat("·", width))
+	}
+
+	maxLen := 0
+	for _, s := range childStatuses {
+		if len(s) > maxLen {
+			maxLen = len(s)
+		}
+	}
+	if maxLen > width {
+		maxLen = width
+	}
+
+	aggregated := make([]bool, maxLen)
+	for i := 0; i < maxLen; i++ {
+		allUp := true
+		for _, statuses := range childStatuses {
+			idx := len(statuses) - maxLen + i
+			if idx >= 0 && !statuses[idx] {
+				allUp = false
+				break
+			}
+		}
+		aggregated[i] = allUp
+	}
+
+	var sb strings.Builder
+	if remaining := width - len(aggregated); remaining > 0 {
+		sb.WriteString(subtleStyle.Render(strings.Repeat("·", remaining)))
+	}
+	for _, up := range aggregated {
+		if up {
+			sb.WriteString(specialStyle.Render("●"))
+		} else {
+			sb.WriteString(dangerStyle.Render("●"))
+		}
+	}
+	return sb.String()
+}
+
+func (m Model) groupUptime(groupID int) string {
+	allSites := m.engine.GetAllSites()
+	var allStatuses [][]bool
+	for _, s := range allSites {
+		if s.ParentID == groupID && !s.Paused && !m.isMonitorInMaintenance(s.ID) {
+			hist, _ := m.engine.GetHistory(s.ID)
+			if len(hist.Statuses) > 0 {
+				allStatuses = append(allStatuses, hist.Statuses)
+			}
+		}
+	}
+	if len(allStatuses) == 0 {
+		return subtleStyle.Render("—")
+	}
+	total, up := 0, 0
+	for _, statuses := range allStatuses {
+		for _, s := range statuses {
+			total++
+			if s {
+				up++
+			}
+		}
+	}
+	return fmtUptime(func() []bool {
+		out := make([]bool, total)
+		idx := 0
+		for _, statuses := range allStatuses {
+			copy(out[idx:], statuses)
+			idx += len(statuses)
+		}
+		return out
+	}())
+}