Fix DMX monitor grid header misalignment

[Copilot]

The header row in DmxMonitorWindow rendered 32 column labels without accounting for the 40px row-label (channel number) at the start of each data row, causing headers to be offset by one column.

Change

Added a 40px empty spacer as the first element of the header row to match the row-label width in data rows:

// Before
EditorGUILayout.BeginHorizontal();
for (int col = 0; col < columns; col++)
    GUILayout.Label($"Column {col + 1}", ..., GUILayout.Width(30));
EditorGUILayout.EndHorizontal();

// After
EditorGUILayout.BeginHorizontal();
GUILayout.Label("", GUILayout.Width(40)); // 行ラベル列のスペーサー
for (int col = 0; col < columns; col++)
    GUILayout.Label($"Column {col + 1}", ..., GUILayout.Width(30));
EditorGUILayout.EndHorizontal();

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[otoyuzu705]

LGTM