feat: gift shop items to another PokeTokenBar user #281

Summary

Allow a user to purchase a shop item as a gift and send it to another
PokeTokenBar user using a temporary pairing code.

Motivation

PokeTokenBar already has a shop and inventory system, but all purchased
items can currently only be used by the purchaser.

Item gifting could add a small social feature without introducing Pokémon
trading or changing the existing companion progression system.

Proposed scope (v1)

• Add a "Gift" option to eligible shop items
• Generate a short-lived pairing code
• Allow another nearby PokeTokenBar user to enter the code
• Add the item to the recipient's inventory
• Deduct the token cost from the sender only after successful delivery
• Prevent the same gift from being redeemed more than once
• Keep the feature local-network only, without accounts or a backend

Explicitly out of scope

• Pokémon trading
• Internet-wide gifting
• Friend accounts or profiles
• Paid items or real-money transactions

Open questions

• Would a local-network-only implementation using Bonjour / Network.framework
  fit the project's on-device privacy direction?
• Should gifting be available for all consumable items, or only selected items
  such as Rare Candy and Mint?
• Would you be open to a PR for this feature?