feat: list pubky marketplace in the shop #1010

Twin: synonymdev/bitkit-android#1570

What

Add a Pubky Marketplace entry to the Shop's Discover screen (Bitkit/Views/Shop/ShopDiscover.swift), next to the existing cards (Gift Cards, ESIMs, Phone Refill, Travel). Tapping it opens the Pubky Marketplace at https://pubky.app/shop.

Expected behavior

• The Shop's Discover screen shows a Pubky Marketplace card with a title, a short description and artwork, in the same style as the other cards.
• Tapping it opens pubky.app/shop, where the user can browse, sign in with Bitkit and pay sellers through Paykit.
• The card's copy is translatable like the other Shop strings.