test: add e2e pagination, property-based, and mutation CI tests

[Idaonoli]

Summary

This PR adds the missing test coverage identified in the four open issues, keeping each change strictly within the scope described.

closes #711 — E2E test for transaction history pagination

e2e/tests/transaction-history.spec.js (new, Playwright)

• Seeds app account state via addInitScript so tests run without a login form.
• Mocks both internal API endpoints (/api/v1/transactions/* and /api/stellar/account/*/transactions) with page.route() — no live Stellar network required.
• Five test cases: initial 20-row load with type/amount/asset assertions; next-page navigation confirming rows change; final-page Next-button disabled state; empty-state illustration visible with no spinner; 503 error showing the error panel and a retry button.
• Bonus continuity test: loads three pages of 20 and asserts all 60 row aria-labels are unique (no duplicate transaction IDs).

closes #712 — Snapshot tests for all design-system components

frontend/tests/snapshots.test.jsx already covers every component in frontend/src/design-system/ (Button — all 4 variants + 3 sizes + fullWidth + disabled + loading; Badge — all 5 variants; Card — header/footer/padding variants; Input — label/hint/error/fullWidth; Modal — all 3 sizes). The snapshot file is committed and CI runs vitest on every PR, so drift fails the build automatically. No code change needed; this issue is resolved by existing committed tests.

closes #713 — Property-based tests for AmountInput validation

property-tests/amountInput.property.test.js (new, fast-check)

• Mirrors the two pure functions from AmountInput.jsx: sanitize() (the handleChange regex pipeline) and formatDisplay() (the Intl.NumberFormat formatter).
• 1000-run properties for: valid decimal strings pass through unchanged; output never contains non-[0-9.] characters; non-numeric chars are always stripped; negative sign is stripped (component rejects negative inputs); multiple dots collapse to one while preserving all digit characters; scientific-notation e is stripped.
• Display-formatter properties: at most 7 decimal places; integers display without a dot; round-trip parse matches original numeric value.
• Boundary cases: Stellar max (922337203685.4775807), zero, and 0.0000000 all sanitize correctly.

closes #714 — Mutation test coverage in CI

.github/workflows/mutation-testing.yml — two changes:

1. Added backend/src/services/** to both push and pull_request path triggers (the stryker mutate scope already covers backend/src/services/*.js, but changes to those files were not triggering the job).
2. Removed continue-on-error: true from the "Run mutation tests" step so that a mutation score below the break: 50 threshold now fails the CI job as intended.

stryker.config.mjs — added inline comments explaining the rationale for each threshold value (break: 50, low: 60, high: 80) per the acceptance criteria.

Test plan

• npx playwright test e2e/tests/transaction-history.spec.js --project=chromium — all 5 specs green
• npm run test:property — amountInput.property.test.js passes all properties with 1000 runs
• Confirm mutation CI job triggers on a PR that touches backend/src/services/ (previously it would not)
• Confirm mutation CI job fails when a mutation causes the score to drop below 50% (previously continue-on-error masked this)
• npm run test — existing snapshot tests in frontend/tests/snapshots.test.jsx continue to pass with committed snapshots

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