fix: encode request query parameters

[Haozhenyu123]

Summary

Fix frontend query parameter encoding for GET requests.

Search and filter values containing reserved characters such as &, +, and # were previously concatenated directly into URLs, causing the backend to receive truncated or incorrect query parameters. For example, searching for ACME & Sons + Partners #1 was parsed as multiple parameters instead of one search value.

Changes

• Add a reusable buildQueryString helper based on URLSearchParams
• Use the helper for filter, search, list, listAll, and summary requests
• Preserve valid falsy values such as 0 and false while omitting null and undefined
• Add dependency-free regression tests using Node's built-in test runner

Testing

• npm test - 3 tests passed
• npm run build - passed
• Prettier check for changed files - passed
• Targeted ESLint check for changed files - passed

Note

The existing npm run lint command fails before linting because .eslintrc.js uses CommonJS while frontend/package.json declares type: module. This pre-existing configuration issue is unrelated to this change.