test: add encryption coverage

[h4yfans]

What

Add dedicated coverage for apps/desktop/src/main/crypto/encryption.ts.

Why

Advance phase 1.2 of the tech-debt remediation plan by giving the crypto encryption layer direct edge-case coverage instead of relying only on broader foundation smoke tests.

How

• add apps/desktop/src/main/crypto/encryption.test.ts
• cover round-trip behavior with and without associated data
• cover tampering, wrong-key, wrong-nonce, AAD mismatch, zero-length payload, and >1 MiB payloads
• cover encryption/decryption error normalization, linking helpers, and unwrapFileKey cleanup behavior

Type

• test — adding or updating tests

Test plan

• Unit tests added/updated
• Integration tests added/updated
• Manual testing (describe below)

Manual verification run locally:

• pnpm --filter @memry/desktop exec vitest run --config config/vitest.config.ts --project main src/main/crypto/encryption.test.ts
• pnpm --filter @memry/desktop exec vitest run --coverage --config config/vitest.config.ts --project main src/main/crypto/encryption.test.ts --coverage.thresholds.statements=0 --coverage.thresholds.branches=0 --coverage.thresholds.functions=0 --coverage.thresholds.lines=0
• pnpm --filter @memry/desktop typecheck:node
• pnpm --filter @memry/desktop exec vitest run --config config/vitest.config.ts --project main src/main/crypto

Checklist

• Self-reviewed the diff
• No hardcoded secrets or credentials
• Files stay under ~500 LOC
• Follows immutable data patterns