#55 Add code coverage reporting

[karanjadavi]

Description:

Problem
We don't know how much of the codebase is covered by tests. Low test coverage means bugs can hide in untested code.
Solution
Add code coverage reporting for both the Rust contract and Next.js frontend.
Tasks

Add cargo-tarpaulin for Rust coverage
Add Jest coverage for frontend
Add coverage reports to CI pipeline
Add coverage badge to README
Set minimum coverage threshold of 80%

Skills needed
Rust, TypeScript, DevOps
Difficulty
Medium