epic: add pubky marketplace to the bitkit shop #1007

Refs: John's request in #bitkit-native

Goal

Bitkit's Shop lists Pubky Marketplace (the name to use; "Pubky Market" is not approved) next to its existing Bitrefill catalog, so users can open the Marketplace from inside Bitkit.

Scope

• Sub-issues for the Bitkit iOS and Android entry points, below.
• Work in other repositories that this needs (Marketplace web, Pubky stack) is added here as further sub-issues or linked PRs.

Open questions

• The entry's design and placement in Shop Discover (Figma).
• The production Marketplace URL to open.