Shopping list: Clear All hides the results container and never shows it again

[dubadub]

Found during review of #456. Pre-existing on main.

clearList() in templates/shopping_list.html adds hidden to #shopping-list-results, but nothing removes it afterwards. After clearing, the "no items" message rendered into #list-content is invisible until the page is reloaded, and adding a recipe from another tab (live updates) renders into a hidden container.

Fix: remove the hidden toggle, or clear it in displayShoppingList().