TAG button shows no feedback when no items are selected

Problem

The TAG button on the /pages/propositions page appears to do nothing when clicked. This is because it requires users to first enable selection mode and select items, but there is no feedback when users click TAG without any selections.

Current Behavior

In pv-tag-button.tsx, when no items are selected, the function silently returns:

if (!selectedItems.length)
    return;  // Silent failure - no user feedback

Expected Behavior

Show a notification/toast message like "Please select items first" when the TAG button is clicked with no selections.

Solution

Add a notification using the existing useNotification hook when the TAG button is clicked without any selected items.