refactor(tui): two-tier responsive table layout (compact/wide at 120 cols)

Replace continuous surplus distribution with two fixed layouts per table.
Breakpoint at 120 columns — matches how btop/k9s do it.

Compact (<120): short headers (LAT, UP%, RT, ST, MON, SENT, VER),
  tight fixed widths, no surplus guessing.

Wide (≥120): full headers (LATENCY, UPTIME, RETRIES, STATUS, MONITORS,
  LAST SENT, VERSION), generous widths.

Sites tab keeps content-aware NAME sizing + sparkline flex.
All other tabs (Alerts, Maint, Nodes, Users) use simple fixed tiers.

Removed old computeTableLayout/colDef/tierCol/pickTier — no longer needed.

internal/tui/tab_nodes.go

@@ -10,14 +10,15 @@ func (m Model) viewNodesTab() string {
 		return "\n  No probe nodes connected."
 	}
 
-	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},
+	var headers []string
+	var widths []int
+	if m.isWide() {
+		headers = []string{"NAME", "REGION", "LAST SEEN", "VERSION", "STATUS"}
+		widths = []int{24, 14, 16, 12, 10}
+	} else {
+		headers = []string{"NAME", "REGION", "SEEN", "VER", "STATUS"}
+		widths = []int{16, 10, 10, 8, 8}
 	}
-	headers, widths := m.computeTableLayout(cols, 0)
 	nameW := widths[0]
 
 	return m.renderTable(