use backend url from env

https://github.com/abhishekbansal2312/BackTest/blob/8d344c972234a8c3b04127b666ead7eddd45465b/src/contexts/BacktestContext.jsx#L158C9-L158C91

Do not keep the url in the code. It should be picked up from the env variables, as it is expected to be changed often.

The issue points to a specific line in src/contexts/BacktestContext.jsx where a backend URL is hardcoded. Look for environment variable configuration in the project (e.g., a .env file or build setup). Replace the hardcoded URL with a reference to the environment variable, ensuring the variable is properly named and loaded. Check if there are any tests or other files that might also use this URL.

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