feat: add local-network item gifting

[kgty0707]

Summary

Adds local-network item gifting without requiring accounts or a backend.

Users can purchase a giftable shop item, generate a temporary pairing code, and send the item to another PokeTokenBar user on the same local network. Giftable items are currently limited to Rare Candy and Mints.

The pairing code expires after one minute and displays a live countdown to the sender. Tokens are deducted only after the recipient has stored the gift successfully. The transfer also prevents the same gift from being redeemed more than once.

Closes #281

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation
• Other:

UI changes

Before

Purchased items could only be added to the buyer's inventory.

After

Giftable shop items now include a gifting flow with a temporary pairing code and a one-minute countdown. Recipients can enter the code to add the item to their inventory.

Demo

Testing

• swift build passes locally.
• The complete build and test suite passes in CI.
• Gift transfer tests cover expiration, duplicate redemption, successful inventory delivery, token deduction, and final-response delivery behavior.
• The required secret scan passes in CI.

Checklist

• swift build and swift test pass locally or in CI
• PR title and description are written in English
• UI changes are described above (before/after — images optional)
• No copyrighted assets, secrets, or private tooling references are committed (see CONTRIBUTING.md)
• Tests were added or updated for this change