Internationalize mobile checkout screen

[mighty840]

Description

The mobile checkout screen (packages/mobile/app/checkout.tsx) has 10+ hardcoded English strings that should use translation keys via react-i18next.

Hardcoded Strings

• "Currently Unavailable" (lines 141, 317)
• "Any special instructions..." (line 204)
• "Enter code" (line 218)
• "Loyalty Points" (line 233)
• "You have X points (100 = $1.00)" (lines 234–235)
• "Points to redeem" (line 238)
• "Contact Information" (line 274)
• "Full name *", "Email address *", "Phone (optional)" (lines 278–280)
• "Loyalty Discount" (line 306)

Steps

1. Add translation keys to the mobile i18n JSON files under a checkout namespace
2. Replace each hardcoded string with t('checkout.keyName')
3. Add translations for all supported languages

Effort

S — repetitive but straightforward