Apply coupon codes at checkout and share feedback in-app

• Apply coupon codes during checkout.
• New in-app feedback dialog so you can share thoughts without leaving the product.
• Account lifecycle emails for signup, out-of-credits, purchase confirmations, plan upgrades, and tier changes.

• Stronger contact form validation, consistent request type design, and better dialog handling.
• Newsletter system now sends acknowledgment emails and internal notifications with cleaner wizard validation.
• Credits now track plan and expiration dates, and consumption logic prefers non-expired same-plan credits.
• Polished plan dialogs, centered credits counter, centered sign-in popup, and a more responsive about page.

Better error messaging for missing user accounts, fixed form layout glitches, and corrected upgrade subscription handling for edge cases.