chore(tui): visual polish — detail sections, column headers, alert detail

Detail panel:
- Grouped fields into sections (ENDPOINT, TIMING, HTTP, CONFIG)
- Omit Timeout when 0 (unconfigured)
- Omit Method when default GET
- Show explicit "200-299" when AcceptedCodes empty

Table:
- LATENCY header → LAT (design short, never truncate)

Alerts:
- Press [i] for alert detail panel: full config, health status,
  send counts, last error
- Keybinding display updated with [i]Info

Bundled remaining UX polish items from screenshot review.

internal/tui/tab_alerts.go

@@ -2,6 +2,7 @@ package tui
 
 import (
 	"fmt"
+	"strings"
 	"time"
 
 	"gitea.lerkolabs.com/lerko/uptop/internal/monitor"
@@ -173,6 +174,52 @@ func (m Model) viewAlertsTab() string {
 	)
 }
 
+func (m Model) viewAlertDetailPanel() string {
+	if m.cursor >= len(m.alerts) {
+		return ""
+	}
+	a := m.alerts[m.cursor]
+	h := m.engine.GetAlertHealth(a.ID)
+
+	var b strings.Builder
+
+	b.WriteString(subtleStyle.Render("  Alerts > ") + titleStyle.Render(a.Name) + "\n\n")
+
+	row := func(label, value string) {
+		fmt.Fprintf(&b, "  %-16s %s\n", subtleStyle.Render(label), value)
+	}
+
+	row("Type", fmtAlertType(a.Type))
+
+	if h.LastSendAt.IsZero() {
+		row("Health", subtleStyle.Render("never sent"))
+	} else if h.LastSendOK {
+		row("Health", specialStyle.Render("OK"))
+	} else {
+		row("Health", dangerStyle.Render("FAILED"))
+	}
+
+	if !h.LastSendAt.IsZero() {
+		row("Last Sent", h.LastSendAt.Format("2006-01-02 15:04:05")+" ("+fmtAlertLastSent(h)+")")
+	}
+	if h.SendCount > 0 {
+		row("Sends", fmt.Sprintf("%d sent, %d failed", h.SendCount, h.FailCount))
+	}
+	if h.LastError != "" {
+		row("Last Error", dangerStyle.Render(limitStr(h.LastError, 60)))
+	}
+
+	b.WriteString("\n" + subtleStyle.Render("  CONFIGURATION") + "\n")
+	for k, v := range a.Settings {
+		row(k, v)
+	}
+
+	b.WriteString("\n\n")
+	b.WriteString(subtleStyle.Render("  [i/Esc] Back  [e] Edit  [t] Test  [q] Quit"))
+
+	return lipgloss.NewStyle().Padding(1, 2).Render(b.String())
+}
+
 func (m *Model) initAlertHuhForm() tea.Cmd {
 	m.alertFormData = &alertFormData{
 		AlertType:         "discord",