ci: add code coverage reporting via Codecov and Tarpaulin

[Francis6-git]

Description:
This PR addresses Issue #344 by implementing comprehensive code coverage reporting across the stack and preserves the WASM optimization workflow required for Stellar/Soroban deployments.

Key Changes

• Backend Coverage: Configured Vitest to generate lcov reports with a mandatory 60% coverage threshold.
• Contract Coverage: Integrated cargo-tarpaulin to generate XML reports for Rust contracts with a mandatory 70% coverage threshold.
• Codecov Integration: Added GitHub Actions steps to upload coverage data for both backend and contracts using codecov-action@v4.
• Build Optimizations: Retained the stellar-cli optimization pass to ensure contract WASM artifacts remain within network limits.
• Visibility: Added a Codecov status badge to the README.md.

CLoses #344

[ogazboiz]

hey, main was failing CI from broken auth imports + frontend parse errors. fixed and pushed to main now. please rebase to pick up the fixes:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

if there's a conflict, resolve it locally and we'll review once CI is green.

[Francis6-git]

hey, main was failing CI from broken auth imports + frontend parse errors. fixed and pushed to main now. please rebase to pick up the fixes:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

if there's a conflict, resolve it locally and we'll review once CI is green.

Conflict resolved

[ogazboiz]

hey, main CI is fully green now. there were several real TypeScript and test issues from #444's merge that have all been fixed. please rebase one more time:

git fetch upstream
git rebase upstream/main
git push --force-with-lease

your PR's diff should be much cleaner after this rebase since main now has the canonical versions of cancel.ts, sorobanService.ts, and the withdraw test.