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_nodes.go

@@ -10,16 +10,24 @@ func (m Model) viewNodesTab() string {
 		return "\n  No probe nodes connected."
 	}
 
-	colWidths := []int{0, 12, 20, 10, 8}
+	cols := []colDef{
+		{"NAME", "NAME", 12, 24, true},
+		{"REGION", "REGION", 8, 14, false},
+		{"SEEN", "LAST SEEN", 8, 20, false},
+		{"VER", "VERSION", 8, 12, false},
+		{"STATUS", "STATUS", 8, 10, false},
+	}
+	headers, widths := m.computeTableLayout(cols, 0)
+	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 +45,7 @@ func (m Model) viewNodesTab() string {
 			}
 			return rows
 		},
-		colWidths,
+		widths,
 		nil,
 	)
 }