Avoid empty initial Plotly render

[rosenrot00]

• remove the empty Plotly.newPlot([], {}) call from the constructor
• let the first real Plotly.react() initialize the plot with actual data and layout
• defer Plotly event listeners until that first render succeeds
• disconnect and reconnect Plotly listeners without duplicates
• support the initial state where Plotly has not created contentEl.data yet

The constructor currently creates a complete empty Plotly graph. The first real render then runs Plotly.react() with the actual traces and layout, so initial card loading performs two Plotly render passes.

Plotly explicitly supports react() as an initial draw. When the target is not a plot div yet, it internally calls newPlot() with the supplied real data. Removing the empty render therefore keeps the same initialization path while avoiding the redundant first pass.

Listeners are attached only after the real plot exists and are guarded against duplicate registration and disconnected cards.