feat(crypto): ✨ Raise symmetric payload limit

[haskou]

Summary

• Raise SymmetricKey payload and ciphertext limit from 1 MiB to 8 MiB.
• Keep asymmetric payload encryption capped at 1 MiB.
• Validate decoded ciphertext size before strict Base64 regex checks to avoid expensive/fragile validation on oversized payloads.
• Update README and technical documentation with separate asymmetric and symmetric limits.

Validation

• yarn lint
• yarn build
• yarn test:coverage (100% statements, branches, functions, and lines)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!