feat(tui): responsive table layout for all tabs

Extract shared computeTableLayout() into table_helpers.go — takes column
definitions with short/full headers, min/max widths, and a flex column
that absorbs surplus space. All tabs now use it:

- Alerts: CONFIG column is flex, NAME/TYPE/SENT expand with width
- Maint: TITLE column is flex, TYPE/MONITORS/STATUS/dates expand
- Nodes: NAME column is flex, REGION/LAST SEEN/VERSION expand
- Users: PUBLIC KEY column is flex, USERNAME expands
- Sites: uses same colDef type (keeps special dual-flex for NAME+HISTORY)

Headers auto-switch short/full based on available width across all tabs.

internal/tui/tab_alerts.go

@@ -148,8 +148,26 @@ func (m Model) viewAlertsTab() string {
 		return "\n  No alert channels configured. Press [n] to add one."
 	}
 
+	maxName := 0
+	for _, a := range m.alerts {
+		if n := len([]rune(a.Name)); n > maxName {
+			maxName = n
+		}
+	}
+
+	cols := []colDef{
+		{"#", "#", 4, 4, false},
+		{"", "", 3, 3, false},
+		{"NAME", "NAME", 10, 20, false},
+		{"TYPE", "TYPE", 10, 12, false},
+		{"CONFIG", "CONFIG", 15, 40, true},
+		{"SENT", "LAST SENT", 8, 12, false},
+	}
+	headers, widths := m.computeTableLayout(cols, 0)
+	nameW := widths[2]
+
 	return m.renderTable(
-		[]string{"#", "", "NAME", "TYPE", "CONFIG", "LAST SENT"},
+		headers,
 		len(m.alerts),
 		func(start, end int) [][]string {
 			var rows [][]string
@@ -159,7 +177,7 @@ func (m Model) viewAlertsTab() string {
 				rows = append(rows, []string{
 					fmt.Sprintf("%d", i+1),
 					fmtAlertHealth(h),
-					m.zones.Mark(fmt.Sprintf("alert-%d", i), limitStr(a.Name, 15)),
+					m.zones.Mark(fmt.Sprintf("alert-%d", i), limitStr(a.Name, nameW-2)),
 					fmtAlertType(a.Type),
 					fmtAlertConfig(struct {
 						Type     string
@@ -170,7 +188,7 @@ func (m Model) viewAlertsTab() string {
 			}
 			return rows
 		},
-		nil, nil,
+		widths, nil,
 	)
 }