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_users.go

@@ -32,8 +32,19 @@ func (m Model) viewUsersTab() string {
 		return "\n  No users configured. Press [n] to add one."
 	}
 
+	var headers []string
+	var widths []int
+	if m.isWide() {
+		headers = []string{"#", "USERNAME", "ROLE", "PUBLIC KEY"}
+		widths = []int{4, 18, 10, 50}
+	} else {
+		headers = []string{"#", "USER", "ROLE", "KEY"}
+		widths = []int{4, 14, 8, 30}
+	}
+	userW := widths[1]
+
 	return m.renderTable(
-		[]string{"#", "USERNAME", "ROLE", "PUBLIC KEY"},
+		headers,
 		len(m.users),
 		func(start, end int) [][]string {
 			var rows [][]string
@@ -41,14 +52,14 @@ func (m Model) viewUsersTab() string {
 				u := m.users[i]
 				rows = append(rows, []string{
 					fmt.Sprintf("%d", i+1),
-					m.zones.Mark(fmt.Sprintf("user-%d", i), limitStr(u.Username, 15)),
+					m.zones.Mark(fmt.Sprintf("user-%d", i), limitStr(u.Username, userW-2)),
 					fmtRole(u.Role),
 					fmtKey(u.PublicKey),
 				})
 			}
 			return rows
 		},
-		nil, nil,
+		widths, nil,
 	)
 }