feat(tui): responsive table columns — expand headers with terminal width

Replace hardcoded column widths with dynamic layout system:
- Each column has short/full header and min/max width
- At narrow terminals: LAT, UP%, RT, compact widths
- At wide terminals: LATENCY, UPTIME, RETRIES, expanded widths
- Surplus space distributed left-to-right across expandable columns
- Headers switch between short/full based on actual column width

Column definitions:
  # (4-6)  TYPE (8-10)  STATUS (8-10)  LAT/LATENCY (5-10)
  UP%/UPTIME (5-8)  SSL (5-7)  RT/RETRIES (5-9)

internal/tui/tab_sites.go

@@ -334,15 +334,50 @@ func fmtDuration(d time.Duration) string {
 	return fmt.Sprintf("%dd", days)
 }
 
-func (m Model) dynamicWidths() (nameW, sparkW int) {
-	fixed := 6 + 10 + 10 + 8 + 8 + 7 + 9 // #, TYPE, STATUS, LATENCY, UPTIME, SSL, RETRY
-	overhead := 30                       // cell padding + borders
-	avail := m.termWidth - chromePadH - 2 - fixed - overhead
-	if avail < 30 {
-		avail = 30
+type tableLayout struct {
+	nameW, sparkW int
+	headers       []string
+	colWidths     []int
+}
+
+func (m Model) computeLayout() tableLayout {
+	type colDef struct {
+		short    string
+		full     string
+		minWidth int
+		maxWidth int
 	}
-	nameW = avail / 2
-	sparkW = avail - nameW - 2 // -2 for spark column padding
+
+	cols := []colDef{
+		{"#", "#", 4, 6},
+		{"", "", 0, 0}, // NAME (dynamic)
+		{"TYPE", "TYPE", 8, 10},
+		{"STATUS", "STATUS", 8, 10},
+		{"LAT", "LATENCY", 5, 10},
+		{"UP%", "UPTIME", 5, 8},
+		{"", "", 0, 0}, // HISTORY (dynamic)
+		{"SSL", "SSL", 5, 7},
+		{"RT", "RETRIES", 5, 9},
+	}
+
+	overhead := 30
+	usable := m.termWidth - chromePadH - 2 - overhead
+	if usable < 80 {
+		usable = 80
+	}
+
+	fixedMin := 0
+	for i, c := range cols {
+		if i == 1 || i == 6 {
+			continue
+		}
+		fixedMin += c.minWidth
+	}
+
+	avail := usable - fixedMin
+	nameW := avail / 2
+	sparkW := avail - nameW - 2
+
 	if nameW < 13 {
 		nameW = 13
 	}
@@ -355,7 +390,50 @@ func (m Model) dynamicWidths() (nameW, sparkW int) {
 	if sparkW > 60 {
 		sparkW = 60
 	}
-	return
+
+	surplus := usable - fixedMin - nameW - sparkW - 2
+	if surplus < 0 {
+		surplus = 0
+	}
+
+	headers := make([]string, len(cols))
+	widths := make([]int, len(cols))
+	for i, c := range cols {
+		if i == 1 {
+			headers[i] = "NAME"
+			widths[i] = 0
+			continue
+		}
+		if i == 6 {
+			headers[i] = "HISTORY"
+			widths[i] = sparkW + 2
+			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
+	}
+
+	return tableLayout{
+		nameW:     nameW,
+		sparkW:    sparkW,
+		headers:   headers,
+		colWidths: widths,
+	}
 }
 
 func (m Model) viewSitesTab() string {
@@ -373,12 +451,13 @@ func (m Model) viewSitesTab() string {
 		return "\n" + welcome
 	}
 
-	nameW, sparkWidth := m.dynamicWidths()
-	colWidths := []int{6, 0, 10, 10, 8, 8, sparkWidth + 2, 7, 9}
+	layout := m.computeLayout()
+	nameW := layout.nameW
+	sparkWidth := layout.sparkW
 
 	var groupRows map[int]bool
 	return m.renderTable(
-		[]string{"#", "NAME", "TYPE", "STATUS", "LAT", "UPTIME", "HISTORY", "SSL", "RETRY"},
+		layout.headers,
 		len(m.sites),
 		func(start, end int) [][]string {
 			groupRows = make(map[int]bool)
@@ -444,7 +523,7 @@ func (m Model) viewSitesTab() string {
 			}
 			return rows
 		},
-		colWidths,
+		layout.colWidths,
 		func(row, col int) *lipgloss.Style {
 			if groupRows[row] {
 				s := siteGroupStyle