fix: prevent duplicate select event dispatch in Dataframe component

[gambletan]

Summary

Fixes #12054

• The select event was being dispatched twice when clicking a Dataframe cell because handle_cell_click was called both on mousedown (in start_drag) and on mouseup (in end_drag) in drag_utils.ts
• Removed the redundant handle_cell_click call from end_drag, since start_drag already handles the cell click and select event dispatch for non-drag (simple click) interactions
• Drag selection behavior is preserved as end_drag only needs to clean up drag state and restore focus

Test plan

• Click on a Dataframe cell and verify the select event handler (fn in .select(fn=...)) is called exactly once
• Verify drag-to-select multiple cells still works correctly
• Verify shift-click and ctrl/cmd-click selection still works
• Test with the reproduction script from the issue to confirm "clicked" is printed once per click

[pngwn]

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