chore(tui): visual polish — detail sections, column headers, alert detail (#37)

## Summary

Bundled remaining UX polish items from the screenshot review.

### Changes

**Detail panel sections (#5)**
- Fields grouped into ENDPOINT, TIMING, HTTP, CONFIG sections with subtle headers
- Matches existing PROBE RESULTS and STATE CHANGES section pattern
- Cleaner visual hierarchy without box-drawing clutter

**Omit unconfigured fields (#6)**
- Timeout hidden when 0 (unconfigured)
- Method hidden when default GET
- AcceptedCodes shows "200-299" explicitly when empty

**Column header (#7)**
- `LATENCY` → `LAT` (design short, never truncate — htop/btop pattern)

**Alert detail view (#8)**
- `i` key on Alerts tab opens full detail panel
- Shows: type, health status, last sent time, send/fail counts, last error
- Full config key:value pairs (untruncated)
- Keybinding: `[i/Esc] Back  [e] Edit  [t] Test  [q] Quit`

### Files (3)
- `internal/tui/tab_sites.go` — section headers, field omission, LAT header
- `internal/tui/tab_alerts.go` — viewAlertDetailPanel()
- `internal/tui/tui.go` — stateAlertDetail, key handler, render routing

Reviewed-on: lerko/uptop#37

internal/tui/tab_nodes.go

@@ -10,16 +10,25 @@ func (m Model) viewNodesTab() string {
 		return "\n  No probe nodes connected."
 	}
 
-	colWidths := []int{0, 12, 20, 10, 8}
+	var headers []string
+	var widths []int
+	if m.isWide() {
+		headers = []string{"NAME", "REGION", "LAST SEEN", "VERSION", "STATUS"}
+		widths = []int{24, 14, 16, 12, 10}
+	} else {
+		headers = []string{"NAME", "REGION", "SEEN", "VER", "STATUS"}
+		widths = []int{16, 10, 10, 8, 8}
+	}
+	nameW := widths[0]
 
 	return m.renderTable(
-		[]string{"NAME", "REGION", "LAST SEEN", "VERSION", "STATUS"},
+		headers,
 		len(m.nodes),
 		func(start, end int) [][]string {
 			var rows [][]string
 			for i := start; i < end; i++ {
 				node := m.nodes[i]
-				name := limitStr(node.Name, 20)
+				name := limitStr(node.Name, nameW-2)
 				if name == "" {
 					name = node.ID
 				}
@@ -37,7 +46,7 @@ func (m Model) viewNodesTab() string {
 			}
 			return rows
 		},
-		colWidths,
+		widths,
 		nil,
 	)
 }