feat: add code coverage reporting via Codecov for backend and contracts

[binayyub4211]

Closes #373

Implemented comprehensive code coverage tracking for both backend (Vitest) and smart contracts (Tarpaulin). Coverage reports are now automatically uploaded to Codecov during CI runs, with enforced thresholds to prevent coverage erosion.

Changes:

Backend: Enabled lcov reporting, set 60% thresholds, and added @vitest/coverage-v8.
Contracts: Integrated cargo-tarpaulin with 70% failure threshold.
CI: Added Codecov upload actions for all coverage reports.
UI/Docs: Added Codecov badge to README and created codecov.yml for PR comments.
Verification:

Verified Vitest configuration structure.
Validated GitHub Actions YAML syntax.
Ensured thresholds match the requirements.

[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.

[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.