Add bulk giftcards extension

[bitkarrot]

Summary

• Adds the Gift Cards extension (bitkarrot/giftcards) to the registry.
• Allows users to create custom bulk gift cards of any kind and any type
• Enables magic link email to claim giftcards using built in SMTP in LNbits
• Create and redeem sats-denominated gift cards with unique secure links.

Extension details

Field	Value
id	giftcards
repo	https://github.com/bitkarrot/giftcards
version	0.2.1
min_lnbits_version	1.5.4
license	MIT

A LICENSE file was added to the giftcards repo and the version bumped to 0.2.1 so the release archive satisfies the registry's mandatory-file checks.

Verification

• extensions.json is valid JSON
• check.py giftcards passes (archive hash matches, icon resolves, min_lnbits_version matches config.json, all mandatory files present in the archive)

[bitkarrot]

Closing in favor of a new PR from a branch on lnbits/lnbits-extensions. The jmeter CI fetches extensions.json via raw.githubusercontent.com/lnbits/lnbits-extensions//extensions.json, which 404s for fork PRs since the branch only exists on the fork. Reopening from a branch pushed directly to this repo so CI can resolve the manifest URL.