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_users.go

@@ -32,13 +32,15 @@ func (m Model) viewUsersTab() string {
 		return "\n  No users configured. Press [n] to add one."
 	}
 
-	cols := []colDef{
-		{"#", "#", 4, 4, false},
-		{"USER", "USERNAME", 10, 18, false},
-		{"ROLE", "ROLE", 8, 10, false},
-		{"KEY", "PUBLIC KEY", 20, 60, true},
+	var headers []string
+	var widths []int
+	if m.isWide() {
+		headers = []string{"#", "USERNAME", "ROLE", "PUBLIC KEY"}
+		widths = []int{4, 18, 10, 50}
+	} else {
+		headers = []string{"#", "USER", "ROLE", "KEY"}
+		widths = []int{4, 14, 8, 30}
 	}
-	headers, widths := m.computeTableLayout(cols, 0)
 	userW := widths[1]
 
 	return m.renderTable(