feat(tui): classify error reasons on DOWN monitors

Categorize raw error strings into DNS/TCP/TLS/HTTP/ICMP/TMO/PRIV
so users get instant triage from the monitor list without opening
the detail panel.

- Status column shows DOWN:DNS, DOWN:TLS, DOWN:HTTP, etc.
- Inline NAME column errors prefixed with category tag [DNS], [TLS]
- Detail panel shows connection chain checklist for HTTP monitors
  (✓ DNS → ✓ TCP → ✗ TLS → · HTTP) pinpointing failure layer
- All display-side only — no database or model changes

internal/tui/view_detail.go

@@ -40,7 +40,8 @@ func (m Model) viewDetailPanel() string {
 		b.WriteString("\n" + subtleStyle.Render("  "+label) + "\n")
 	}
 
-	row("Status", fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID)))
+	errCat := classifyError(site.LastError, site.Type, site.StatusCode)
+	row("Status", fmtStatus(site.Status, site.Paused, m.isMonitorInMaintenance(site.ID), errCat))
 
 	if (site.Status == "DOWN" || site.Status == "SSL EXP" || site.Status == "LATE") && site.LastError != "" {
 		row("Error", dangerStyle.Render(limitStr(site.LastError, 60)))
@@ -50,6 +51,34 @@ func (m Model) viewDetailPanel() string {
 		row("HTTP Code", strconv.Itoa(site.StatusCode))
 	}
 
+	if (site.Status == "DOWN" || site.Status == "SSL EXP") && site.LastError != "" {
+		chain := connectionChain(site.LastError, site.Type, site.StatusCode, strings.HasPrefix(site.URL, "https"))
+		if len(chain) > 0 {
+			b.WriteString("\n")
+			for _, step := range chain {
+				var icon string
+				switch step.Status {
+				case stepPassed:
+					icon = specialStyle.Render("✓")
+				case stepFailed:
+					icon = dangerStyle.Render("✗")
+				case stepSkipped:
+					icon = subtleStyle.Render("·")
+				}
+				line := fmt.Sprintf("  %s %-16s", icon, step.Name)
+				if step.Detail != "" {
+					switch step.Status {
+					case stepFailed:
+						line += " " + dangerStyle.Render(step.Detail)
+					case stepSkipped:
+						line += " " + subtleStyle.Render(step.Detail)
+					}
+				}
+				b.WriteString(line + "\n")
+			}
+		}
+	}
+
 	if !site.StatusChangedAt.IsZero() {
 		dur := time.Since(site.StatusChangedAt)
 		row("State Since", site.StatusChangedAt.Format("2006-01-02 15:04:05")+" ("+fmtDuration(dur)+")")