refactor(tui): extract shared table rendering, fix cursor bounds

- New table_helpers.go with renderTable() and shared styles
- Remove 4 duplicated style blocks (header/cell/selected/border)
  from tab_alerts.go and tab_users.go
- All 3 tab views now use renderTable() for offset/end calc,
  selected row highlighting, and table construction
- Sites tab keeps siteGroupStyle via StyleOverride callback
- Clamp cursor to list length at end of refreshData() to prevent
  index-out-of-bounds after concurrent list changes
- Fix off-by-one in tab click handler (i <= maxTabs → i < tabCount)

internal/tui/table_helpers.go

@@ -0,0 +1,75 @@
+package tui
+
+import (
+	"github.com/charmbracelet/lipgloss"
+	"github.com/charmbracelet/lipgloss/table"
+)
+
+var (
+	tableHeaderStyle = lipgloss.NewStyle().
+				Foreground(lipgloss.Color("#7D56F4")).
+				Bold(true).
+				Padding(0, 1)
+
+	tableCellStyle = lipgloss.NewStyle().Padding(0, 1)
+
+	tableSelectedStyle = lipgloss.NewStyle().
+				Padding(0, 1).
+				Bold(true).
+				Foreground(lipgloss.Color("#ffffff")).
+				Background(lipgloss.Color("#3b3b5c"))
+
+	tableBorderStyle = lipgloss.NewStyle().
+				Foreground(lipgloss.Color("#444"))
+)
+
+type StyleOverride func(row, col int) *lipgloss.Style
+
+func (m Model) renderTable(headers []string, items int, buildRows func(start, end int) [][]string, colWidths []int, styleOverride StyleOverride) string {
+	if items == 0 {
+		return ""
+	}
+
+	end := m.tableOffset + m.maxTableRows
+	if end > items {
+		end = items
+	}
+
+	selectedVisual := m.cursor - m.tableOffset
+	rows := buildRows(m.tableOffset, end)
+
+	tableWidth := m.termWidth - 6
+	if tableWidth < 40 {
+		tableWidth = 40
+	}
+
+	t := table.New().
+		Border(lipgloss.RoundedBorder()).
+		BorderStyle(tableBorderStyle).
+		Width(tableWidth).
+		Headers(headers...).
+		Rows(rows...).
+		StyleFunc(func(row, col int) lipgloss.Style {
+			if row == table.HeaderRow {
+				return tableHeaderStyle
+			}
+			if styleOverride != nil {
+				if s := styleOverride(row, col); s != nil {
+					if col < len(colWidths) && colWidths[col] > 0 {
+						return s.Width(colWidths[col])
+					}
+					return *s
+				}
+			}
+			base := tableCellStyle
+			if row == selectedVisual {
+				base = tableSelectedStyle
+			}
+			if col < len(colWidths) && colWidths[col] > 0 {
+				base = base.Width(colWidths[col])
+			}
+			return base
+		})
+
+	return "\n" + t.Render()
+}