fix: show specific error messages for waitlist registration (#673)

[NewCoder3294]

Summary

• Improved error handling for the "Reserve Your Spot" waitlist registration
• Users now see specific messages instead of generic "Registration failed" for:
  • Rate limiting (429) -> "Too many attempts. Please try again in an hour."
  • Service unavailable (503) -> "Registration service is temporarily unavailable."
  • Network errors (fetch failure) -> "Network error. Check your connection and try again."
• Added i18n keys (errorRateLimit, errorServiceUnavailable, errorNetwork) in en.json
• Other locales fall back to English via i18next fallbackLng: 'en'

Test plan

• Register with new email -> success message
• Register with same email again -> "already registered" message
• Simulate rate limiting (6+ requests in 1 hour) -> "Too many attempts" message
• Simulate service unavailable (503) -> "Registration service is temporarily unavailable"
• Disconnect network, then register -> "Network error" message
• TypeScript type check passes (npx tsc --noEmit)

Closes #673

🤖 Generated with Claude Code

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