feat(datatables): add Column::visible() for conditional column visibility

[sebastienheyd]

Summary

• Add Column::visible(bool) method to dynamically toggle column visibility based on request parameters (e.g., ->visible(request()->post('showMine', false)))
• Simplify Column::hidden() to delegate to visible(false)
• Guard against columns without data in Datatable::make() to prevent errors with special columns

Closes #92

Usage

Instead of conditionally adding/removing columns (which causes "column not found" errors), always define all columns and use visible():

public function columns(): array
{
    return [
        Column::add('ID')->data('id'),
        Column::add('Name')->data('name')->visible(request()->post('showMine', false)),
    ];
}

Test plan

• Column::visible(true) does not add visible to attributes
• Column::visible(false) sets visible: false
• Column::hidden() still works as before
• Column::hidden()->visible(true) removes the hidden state
• All 232 tests pass

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