Fix referral code links on preview deployments

[meteop-x]

Summary

• Build /api/referrals/code links from NEXT_PUBLIC_APP_URL when configured, otherwise from the current request origin.
• URL-encode the referral code before putting it into the invite link.
• Add regression coverage for preview-origin links and configured app URL links.

Fixes #127.

uGig gig: https://ugig.net/gigs/abd6b2a0-e728-48cf-a46f-f99e419ed94e

Validation

• node_modules\.bin\vitest.cmd run src/app/api/referrals/code/route.test.ts
• node_modules\.bin\tsc.cmd --noEmit
• node_modules\.bin\prettier.cmd --check src/app/api/referrals/code/route.ts src/app/api/referrals/code/route.test.ts
• git diff --check

Note: corepack pnpm install --frozen-lockfile populated dependencies, but the repo postinstall script fails on this Windows shell because it invokes bare pnpm and Unix true; direct local binaries were used for validation after install.

[meteop-x]

Closing this to avoid a duplicate submission: PR #128 already covers issue #127. I'll move to a non-overlapping bug instead.