Clarify coupon redeem rules and fix Chromium path types

[kriszerops]

Summary

• Spell out that the workshop code can be used on a first top-up or redeemed later on its own, and that coin codes add credit with no payment.
• Drop the hardcoded $10 top-up amount so the copy stays correct for larger verification payments.
• Fix resolveChromiumPath so TypeScript no longer passes undefined into existsSync.

Test plan

• Open / and check the coupon section: two small lines with ticket/coin emojis, no $10.
• Download the coupon image and confirm the same notes in the footer.
• npm run build succeeds.