Add bottom shadow to dealer list for scroll affordance

Description

The dealer list on the Where to Buy page currently looks visually "cut off" at the bottom when there are more items than fit in the visible area. Adding a subtle shadow/gradient fade at the bottom would:

1. Indicate scrollability — visually hint to users that more content exists below
2. Improve aesthetics — soften the hard cutoff for a more polished look

Current behavior

The list ends abruptly at the container boundary (lg:max-h-[560px] lg:overflow-y-auto), giving no visual cue that more dealers are available below.

<img width="549" height="701" alt="Image" src="https://github.com/user-attachments/assets/4769ac9c-224e-4ea8-998d-da2c63012222" />

Proposed change

Add a bottom shadow overlay (e.g. a gradient from transparent to the background color) on the scrollable dealer list container. The shadow should:

• Only appear when there is more content to scroll (hide it when scrolled to the bottom)
• Use the existing dark background color for the gradient

Files involved

• storefront/components/where-to-buy/where-to-buy-content.tsx — the scrollable parent container (line ~109)
• storefront/components/where-to-buy/dealer-list.tsx — the dealer list component

Labels

Enhancement, UI