Reduce web app memory usage during import/export and vault updates

[lanedirt]

Vault encryption and decryption during load and save can cause a high memory usage, as it requires to hold several full-size copies of the vault at once for various operations.

For large vaults this can exhaust the WASM runtime's memory and throw an OutOfMemoryException in the browser.

Todo:

• Optimize encryption and decryption paths for efficiency and prevent storing more objects in memory than absolutely necessary