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/view_detail.go

@@ -0,0 +1,207 @@
+package tui
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/charmbracelet/lipgloss"
+)
+
+func (m Model) viewDetailPanel() string {
+	if m.cursor >= len(m.sites) {
+		return ""
+	}
+	site := m.sites[m.cursor]
+	hist, _ := m.engine.GetHistory(site.ID)
+
+	var b strings.Builder
+
+	var breadcrumb string
+	if site.ParentID > 0 {
+		for _, s := range m.sites {
+			if s.ID == site.ParentID {
+				breadcrumb = subtleStyle.Render("  Sites > "+s.Name+" > ") + titleStyle.Render(site.Name)
+				break
+			}
+		}
+	}
+	if breadcrumb == "" {
+		breadcrumb = subtleStyle.Render("  Sites > ") + titleStyle.Render(site.Name)
+	}
+	b.WriteString(breadcrumb + "\n\n")
+
+	row := func(label, value string) {
+		fmt.Fprintf(&b, "  %-16s %s\n", subtleStyle.Render(label), value)
+	}
+
+	section := func(label string) {
+		b.WriteString("\n" + subtleStyle.Render("  "+label) + "\n")
+	}
+
+	row("Status", fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)))
+
+	if (site.Status == "DOWN" || site.Status == "SSL EXP" || site.Status == "LATE") && site.LastError != "" {
+		row("Error", dangerStyle.Render(limitStr(site.LastError, 60)))
+	}
+
+	if site.Type == "http" && site.StatusCode > 0 {
+		row("HTTP Code", strconv.Itoa(site.StatusCode))
+	}
+
+	if !site.StatusChangedAt.IsZero() {
+		dur := time.Since(site.StatusChangedAt)
+		row("State Since", site.StatusChangedAt.Format("2006-01-02 15:04:05")+" ("+fmtDuration(dur)+")")
+	}
+
+	if !site.LastSuccessAt.IsZero() {
+		ago := time.Since(site.LastSuccessAt)
+		row("Last Success", site.LastSuccessAt.Format("15:04:05")+" ("+fmtDuration(ago)+" ago)")
+	}
+
+	if m.isMonitorInMaintenance(site.ID) {
+		for _, mw := range m.maintenanceWindows {
+			if mw.Type == "maintenance" && (mw.MonitorID == 0 || mw.MonitorID == site.ID || mw.MonitorID == site.ParentID) {
+				row("Maintenance", maintStyle.Render(mw.Title))
+				break
+			}
+		}
+	}
+
+	section("ENDPOINT")
+	row("Type", site.Type)
+	if site.URL != "" {
+		row("URL", site.URL)
+	}
+	if site.Hostname != "" {
+		row("Host", site.Hostname)
+	}
+	if site.Port > 0 {
+		row("Port", strconv.Itoa(site.Port))
+	}
+
+	section("TIMING")
+	row("Interval", fmt.Sprintf("%ds", site.Interval))
+	if site.Timeout > 0 {
+		row("Timeout", fmt.Sprintf("%ds", site.Timeout))
+	}
+	row("Latency", fmtLatency(site.Latency))
+	row("Uptime", fmtUptime(hist.Statuses))
+	if !site.LastCheck.IsZero() {
+		row("Last Check", site.LastCheck.Format("15:04:05"))
+	}
+
+	if site.Type == "http" {
+		section("HTTP")
+		if site.Method != "" && site.Method != "GET" {
+			row("Method", site.Method)
+		}
+		codes := site.AcceptedCodes
+		if codes == "" {
+			codes = "200-299"
+		}
+		row("Codes", codes)
+		row("SSL", fmtSSL(site))
+		if site.IgnoreTLS {
+			row("TLS Verify", dangerStyle.Render("disabled"))
+		}
+	}
+
+	if site.MaxRetries > 0 || site.Regions != "" || site.Description != "" {
+		section("CONFIG")
+		if site.MaxRetries > 0 {
+			row("Retries", fmtRetries(site))
+		}
+		if site.Regions != "" {
+			row("Regions", site.Regions)
+		}
+		if site.Description != "" {
+			row("Description", site.Description)
+		}
+	}
+
+	probeResults := m.engine.GetProbeResults(site.ID)
+	if len(probeResults) > 0 {
+		b.WriteString("\n" + subtleStyle.Render("  PROBE RESULTS") + "\n")
+		for nodeID, result := range probeResults {
+			status := specialStyle.Render("UP")
+			if !result.IsUp {
+				status = dangerStyle.Render("DN")
+			}
+			latency := time.Duration(result.LatencyNs).Milliseconds()
+			ago := time.Since(result.CheckedAt).Truncate(time.Second)
+			line := fmt.Sprintf("  %-14s %s  %dms  %s ago", nodeID, status, latency, ago)
+			if !result.IsUp && result.ErrorReason != "" {
+				line += "  " + dangerStyle.Render(limitStr(result.ErrorReason, 30))
+			}
+			b.WriteString(line + "\n")
+		}
+	}
+
+	stateChanges := m.engine.GetStateChanges(site.ID, 5)
+	if len(stateChanges) > 0 {
+		b.WriteString("\n" + subtleStyle.Render("  STATE CHANGES") + "\n")
+		for _, sc := range stateChanges {
+			ago := fmtDuration(time.Since(sc.ChangedAt))
+			arrow := subtleStyle.Render(sc.FromStatus) + " → "
+			if sc.ToStatus == "UP" {
+				arrow += specialStyle.Render(sc.ToStatus)
+			} else {
+				arrow += dangerStyle.Render(sc.ToStatus)
+			}
+			line := fmt.Sprintf("  %s  %s", arrow, subtleStyle.Render(ago+" ago"))
+			if sc.ErrorReason != "" && sc.ToStatus != "UP" {
+				line += "  " + dangerStyle.Render(limitStr(sc.ErrorReason, 40))
+			}
+			b.WriteString(line + "\n")
+		}
+	}
+
+	b.WriteString("\n")
+	const sparkWidth = 40
+	if site.Type == "push" {
+		b.WriteString("  " + heartbeatSparkline(hist.Statuses, sparkWidth))
+		if len(hist.Statuses) > 0 {
+			up := 0
+			for _, s := range hist.Statuses {
+				if s {
+					up++
+				}
+			}
+			fmt.Fprintf(&b, "\n  %s %d/%d checks up",
+				subtleStyle.Render("Heartbeats"),
+				up, len(hist.Statuses))
+		}
+	} else {
+		b.WriteString("  " + latencySparkline(hist.Latencies, hist.Statuses, sparkWidth))
+		var minL, maxL, total time.Duration
+		count := 0
+		for i, l := range hist.Latencies {
+			if i < len(hist.Statuses) && !hist.Statuses[i] {
+				continue
+			}
+			if count == 0 {
+				minL, maxL = l, l
+			} else if l < minL {
+				minL = l
+			} else if l > maxL {
+				maxL = l
+			}
+			total += l
+			count++
+		}
+		if count > 0 {
+			avg := total / time.Duration(count)
+			fmt.Fprintf(&b, "\n  %s %dms  %s %dms  %s %dms",
+				subtleStyle.Render("Min"), minL.Milliseconds(),
+				subtleStyle.Render("Avg"), avg.Milliseconds(),
+				subtleStyle.Render("Max"), maxL.Milliseconds())
+		}
+	}
+
+	b.WriteString("\n\n")
+	b.WriteString(subtleStyle.Render("  [i/Esc] Back  [e] Edit  [q] Quit"))
+
+	return lipgloss.NewStyle().Padding(1, 2).Render(b.String())
+}