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Availability Grid
A weekly grid to share your free time: paint slots by dragging (mouse or finger), erase them by starting from a taken slot; with the keyboard, arrows move through the grid and Space toggles a slot; a readable summary (Mon 9–12, Wed 14–17) and the total hours update live.
No dependency: paste the HTML into your page, the CSS into your stylesheet, and the JavaScript into a function that receives the parent element (root).