Add comprehensive unit tests for frontend certificate.js

[Hemavathi15sg]

Summary

Add thorough unit test coverage for frontend/certificate.js to ensure certificate verification flows are resilient, including edge cases, API failures, and invalid data scenarios.

Scope

• Cover success and failure paths for certificate verification logic (including fetch/HTTP error handling, non-200 responses, and malformed JSON).
• Validate behavior when inputs are missing/invalid (empty certificate ID/serial, whitespace-only, excessively long values, special characters).
• Ensure UI state updates are tested (loading indicators, error/success messages, button disabled states, cleared/retained inputs on submit/retry).
• Test handling of timeout/abort scenarios (e.g., fetch aborted or delayed responses) if supported in code.
• Verify rendering of certificate details uses safe defaults when optional fields are absent (expiry date null, missing holder/course fields).
• Add regression tests for QR/verification URL rendering if applicable in certificate.js.

Approach

• Use a JS test runner suitable for the frontend bundle (e.g., Jest + jsdom, or the project’s preferred runner) and mock fetch and DOM APIs.
• Add fixtures for valid/invalid certificate payloads and error responses.
• Include negative tests: network failure, 500s/404s, invalid JSON, signature mismatch payloads if applicable.
• Ensure tests run via CI (document command in README if new).

Definition of Done

• New tests added under frontend/ (or existing tests folder) with clear arrange/act/assert structure.
• Tests deterministic and not relying on live network calls.
• Test command documented if new.
• CI passes with added coverage improvements noted.

/cc @CanarysPlayground