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_users.go

@@ -32,8 +32,17 @@ func (m Model) viewUsersTab() string {
 		return "\n  No users configured. Press [n] to add one."
 	}
 
+	cols := []colDef{
+		{"#", "#", 4, 4, false},
+		{"USER", "USERNAME", 10, 18, false},
+		{"ROLE", "ROLE", 8, 10, false},
+		{"KEY", "PUBLIC KEY", 20, 60, true},
+	}
+	headers, widths := m.computeTableLayout(cols, 0)
+	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 +50,14 @@ 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,
 	)
 }