Use STATUSLINE_WIDTH env var for reliable width detection

term.GetSize returns unreliable values when running inside Claude Code
over SSH. Replace the hardcoded statuslineWidthOffset with STATUSLINE_WIDTH
env var (set in Claude Code settings), falling back to terminal detection,
then 80 columns.

main_test.go

@@ -756,10 +756,9 @@ func TestTokenUsage_ZeroValues(t *testing.T) {
 	}
 }
 
-func TestStatuslineWidthOffset_Constant(t *testing.T) {
-	// Verify the constant is defined and reasonable
-	if statuslineWidthOffset <= 0 || statuslineWidthOffset > 20 {
-		t.Errorf("statuslineWidthOffset = %d, expected between 1 and 20", statuslineWidthOffset)
+func TestDefaultStatuslineWidth_Constant(t *testing.T) {
+	if defaultStatuslineWidth < 40 || defaultStatuslineWidth > 300 {
+		t.Errorf("defaultStatuslineWidth = %d, expected between 40 and 300", defaultStatuslineWidth)
 	}
 }