fix: persist birthday cake draft state

[saurabhhhcodes]

Summary

• Persist the birthday cake draft selection in localStorage so flavor, weight, and message survive navigation/refresh
• Restore the saved birthday draft after the product catalog loads so the image, price, active buttons, and message input stay in sync
• Add semantic classes to the existing flavor/weight buttons so restored state is reflected visually

Validation

• node --check public/script.js
• git diff --check
• npm ci is blocked by the existing lockfile drift: package-lock.json is missing fsevents@2.3.3
• npm test was attempted after a local npm install, but the current suite fails before this frontend change because tests destructure { app } from api/index.js while the module exports the Express app directly, so Supertest receives undefined; api/index.js also starts a real server during test import

Closes #629

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