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_maint.go

@@ -100,16 +100,15 @@ func (m Model) viewMaintTab() string {
 		return "\n  No maintenance windows or incidents. Press [n] to create one."
 	}
 
-	cols := []colDef{
-		{"#", "#", 4, 4, false},
-		{"TITLE", "TITLE", 12, 24, false},
-		{"TYPE", "TYPE", 13, 14, false},
-		{"MON", "MONITORS", 14, 22, true},
-		{"ST", "STATUS", 11, 12, false},
-		{"START", "STARTED", 14, 16, false},
-		{"ENDS", "ENDS", 14, 16, false},
+	var headers []string
+	var widths []int
+	if m.isWide() {
+		headers = []string{"#", "TITLE", "TYPE", "MONITORS", "STATUS", "STARTED", "ENDS"}
+		widths = []int{4, 24, 14, 22, 12, 16, 16}
+	} else {
+		headers = []string{"#", "TITLE", "TYPE", "MON", "ST", "START", "ENDS"}
+		widths = []int{4, 14, 13, 14, 11, 14, 14}
 	}
-	headers, widths := m.computeTableLayout(cols, 0)
 	titleW := widths[1]
 	monW := widths[3]
 	timeW := widths[5]