feat: Last Seen column tracking Alertmanager re-send heartbeat

The alert list showed only Started, which comes from Prometheus and
never changes for the lifetime of an alert instance. A firing alert
that started 12 days ago looked identical whether Alertmanager
refreshed it 30 seconds ago or went silent a week ago.

terdut-server already tracks this: the webhook upsert sets
received_at on every accepted payload, including the periodic
re-sends issued at repeat_interval, and its archiver treats the
field as a liveness heartbeat. The field was already decoded into
api.Alert.ReceivedAt and simply never rendered.

Add a Last Seen column to the alert tables, rendered with the
existing humanAgo helper. The Alerts and Archived tabs share
alertColumns/alertRows, so both pick it up. The width budget is
re-derived for five columns; the slack constant now accounts for
all of bubbles' per-cell padding, so the table lands exactly on
the terminal width instead of overflowing by two columns as it
did with four.

The detail view gains a matching Last Seen line, with the timeline
labels widened to keep values aligned. Since received_at stops
advancing once an alert resolves, also pull through the server's
resolution_source and show it in the status header
(RESOLVED · alertmanager vs RESOLVED · expiry) so a frozen
timestamp is explained.

internal/tui/view.go

@@ -381,7 +381,11 @@ func buildDetailContent(alert api.Alert, comments []api.Comment, cursor, width i
 	if alert.Status == "firing" {
 		statusStr = styleFiring.Render("● FIRING")
 	} else {
-		statusStr = styleResolved.Render("✓ RESOLVED")
+		label := "✓ RESOLVED"
+		if alert.ResolutionSource != nil {
+			label += " · " + *alert.ResolutionSource
+		}
+		statusStr = styleResolved.Render(label)
 	}
 	nameGap := contentW - lipgloss.Width(name) - lipgloss.Width(statusStr)
 	if nameGap < 1 {
@@ -390,17 +394,19 @@ func buildDetailContent(alert api.Alert, comments []api.Comment, cursor, width i
 	b.WriteString("\n  " + name + strings.Repeat(" ", nameGap) + statusStr + "\n\n")
 
 	// Timeline
-	b.WriteString(fmt.Sprintf("  Started:  %s  (%s)\n",
+	b.WriteString(fmt.Sprintf("  Started:    %s  (%s)\n",
 		alert.StartsAt.UTC().Format("2006-01-02 15:04 UTC"), humanAgo(now, alert.StartsAt)))
+	b.WriteString(fmt.Sprintf("  Last Seen:  %s  (%s)\n",
+		alert.ReceivedAt.UTC().Format("2006-01-02 15:04 UTC"), humanAgo(now, alert.ReceivedAt)))
 	if alert.EndsAt != nil {
-		b.WriteString(fmt.Sprintf("  Ended:    %s\n", alert.EndsAt.UTC().Format("2006-01-02 15:04 UTC")))
+		b.WriteString(fmt.Sprintf("  Ended:      %s\n", alert.EndsAt.UTC().Format("2006-01-02 15:04 UTC")))
 	}
 	if alert.GeneratorURL != "" {
 		url := alert.GeneratorURL
 		if len(url) > contentW-12 {
 			url = url[:contentW-15] + "…"
 		}
-		b.WriteString(fmt.Sprintf("  Source:   %s\n", url))
+		b.WriteString(fmt.Sprintf("  Source:     %s\n", url))
 	}
 	b.WriteString("\n")