[Test Improver] Add unit tests for crypto/encryption utilities

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🤖 This PR was created by Test Improver, an automated AI assistant focused on improving test coverage.

Goal and Rationale

shell/utils/crypto/encryption.ts provides the deriveKey, encrypt, and decrypt functions that are the cryptographic backbone of the notification store — it encrypts sensitive notification data before persisting it to localStorage. Despite being security-critical code, it had zero test coverage.

Approach

Wrote 15 unit tests across three describe blocks:

Describe	What is tested
deriveKey	Algorithm name (AES-GCM), non-extractability, correct key usages, per-password uniqueness (verified by cross-key decryption failure)
encrypt	Output shape (cipher + iv), base64 format, random IV per call
decrypt	Round-trip fidelity for plain text, unicode, long strings, JSON, special characters; rejection on wrong key; rejection on tampered cipher

The JSDOM test environment doesn't ship the Web Crypto API, so the test file wires up Node.js's built-in webcrypto via Object.defineProperty(global, 'crypto', { value: webcrypto }) — a pattern limited to this single test file.

Coverage Impact

File	Before	After
shell/utils/crypto/encryption.ts	0 %	~100 %

Test Status

All 15 tests pass:

PASS shell/utils/crypto/__tests__/encryption.test.ts
Tests: 15 passed, 15 total

Lint: no errors or warnings (eslint --max-warnings 0).

Trade-offs

• The PBKDF2 key derivation runs 100,000 iterations, so tests involving deriveKey take ~100–200 ms each. This is acceptable for 15 tests and reflects the real cost of the function.
• Tampered-cipher test appends base64 characters to the ciphertext; this changes the decoded byte length which AES-GCM rejects (integrity check fails).

Reproducibility

yarn test:ci shell/utils/crypto/__tests__/encryption.test.ts

Warning

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• download.cypress.io

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