Function SelectBooksPicker

A searchable dropdown (combobox) for picking multiple books from the Bible canon. It provides:

• A trigger button summarizing how many books are selected
• A searchable list of all available books, grouped by section
• "Select all" / "Clear all" shortcuts
• Support for shift-click and shift-Enter range selection

This is the standalone picker used by SelectBooks, which additionally renders section quick-select buttons and badges for the current selection.