#58 Add Stellar mainnet deployment

[karanjadavi]

Description:

Problem
StellarProof is currently only on testnet. For real users to use it the contract needs to be deployed to Stellar mainnet.
Solution
Deploy StellarProof to Stellar mainnet after a security audit and testing phase.
Tasks

Complete security audit
Set up mainnet Stellar account
Deploy contract to mainnet
Update frontend to use mainnet
Add mainnet/testnet toggle
Announce mainnet launch

Skills needed
Rust, Soroban, Stellar CLI
Difficulty
Hard