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/table_helpers.go

@@ -15,6 +15,79 @@ var (
 
 type StyleOverride func(row, col int) *lipgloss.Style
 
+type colDef struct {
+	short    string
+	full     string
+	minWidth int
+	maxWidth int
+	flex     bool
+}
+
+func (m Model) computeTableLayout(cols []colDef, maxContentWidth int) ([]string, []int) {
+	numCols := len(cols)
+	borderOverhead := 2 + (numCols - 1)
+	usable := m.termWidth - chromePadH - 2 - borderOverhead
+	if usable < 40 {
+		usable = 40
+	}
+
+	fixedMin := 0
+	flexIdx := -1
+	for i, c := range cols {
+		if c.flex {
+			flexIdx = i
+			continue
+		}
+		fixedMin += c.minWidth
+	}
+
+	flexW := usable - fixedMin
+	if maxContentWidth > 0 && flexW > maxContentWidth {
+		flexW = maxContentWidth
+	}
+	if flexW < 8 {
+		flexW = 8
+	}
+
+	surplus := usable - fixedMin - flexW
+	if surplus < 0 {
+		surplus = 0
+	}
+
+	headers := make([]string, numCols)
+	widths := make([]int, numCols)
+	for i, c := range cols {
+		if c.flex {
+			headers[i] = c.full
+			widths[i] = flexW
+			continue
+		}
+
+		w := c.minWidth
+		expand := c.maxWidth - c.minWidth
+		if surplus >= expand {
+			w = c.maxWidth
+			surplus -= expand
+		} else if surplus > 0 {
+			w += surplus
+			surplus = 0
+		}
+
+		if w >= len(c.full)+2 {
+			headers[i] = c.full
+		} else {
+			headers[i] = c.short
+		}
+		widths[i] = w
+	}
+
+	if surplus > 0 && flexIdx >= 0 {
+		widths[flexIdx] += surplus
+	}
+
+	return headers, widths
+}
+
 func (m Model) renderTable(headers []string, items int, buildRows func(start, end int) [][]string, colWidths []int, styleOverride StyleOverride) string {
 	if items == 0 {
 		return ""