92 add automated tests for critical functionality

[ludvigz]

Purpose

Core functionality (compatibility checks, build persistence, GraphQL resolvers and build publishing) had little or no automated test coverage, which made regressions hard to catch. This PR adds tests for the most important parts, prioritising compatibility checks and backend validation.

Changes

• Add checkCompatability.test.ts: compatible and incompatible combinations for CPU/motherboard, RAM, case, GPU, PSU, cooler and storage, plus missing, negative and non-numeric data staying "unknown", an empty build, and several parts in one category.
• Add BuildProvider.test.tsx: loading saved builds, handling of invalid or corrupted localStorage data (broken JSON, empty values, null, non-object JSON, the browser refusing to read or save), and adding, replacing, removing and clearing parts.
• Fix a bug found by those tests: BuildProvider accepted valid JSON that was not an object (for example a string or array) as the saved build. It now only accepts a plain object.
• Add publishBuild.test.ts: valid publishing (trimming, length limits, parts stored in the right columns, user input kept out of the SQL) and ten kinds of invalid requests, each verified to save nothing.
• Add catalogResolvers.test.ts: the parts resolver (pagination cursors, page-size limits, filters, sorting, attribute filters), part, manufacturers, builds, and the Part/Build field resolvers.
• The backend tests mock the database and config.js (the config loads dotenv and needs DATABASE_URL, which tests do not have).
• Document the backend test command (npm test in backend/server) in the README.

Related issues

Closes #92

Verification

Automated checks:

• backend/server: npm test passes all 76 tests (46 new in this PR).
• frontend: npm test passes all 81 tests (50 new).

Screenshots

Not applicable

Checklist

• I have reviewed my own changes.
• The changes meet the linked issue's acceptance criteria, where applicable.
• I have verified the changes and recorded the results above.
• I have added or updated tests where needed.
• I have updated documentation where needed.
• I have included screenshots for visual changes, or marked them not applicable.

[oscarog]

Nice work!