Wrong Frontend Calculation in Preview during Create Subscription flow

[hiteshshimpi-55]

Problem

• As we continue to add more entities within a subscription, the calculation logic on the frontend is increasingly diverging from the backend.
• This results in inconsistent values being displayed to users and introduces room for errors, especially as the logic becomes more complex.

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Goal

Define a single source of truth for subscription-related calculations that can be consistently applied across both frontend and backend. This could involve introducing a shared rules engine (e.g., JSON-based rules or configuration-driven approach) to ensure all calculations remain accurate, maintainable, and easy to extend in the future. Open for discussion.

[atharvamhaske]

I went through code and i found that its due to tax calculation i have seen comment that says we in test mode use 10% tax and we have to use proper tax percent from tax-api, also The old logic in Preview.tsx applied discounts first, then tax which can be one of cause.
cc: @hiteshshimpi-55