Feature: end-to-end encryption for conversations

[think-in-universe]

Implement end-to-end encryption for Private Chat with NEAR MPC + Cloud API.

As now we have supported end-to-end encryption in Cloud API, we can implement the end-to-end encryption for any users who can safely manage their private key, e.g. with NEAR MPC if the user has a NEAR account.

The encrypted chat process for streaming

1. The user logins with NEAR wallet;
2. Request a private key with NEAR MPC;
3. Encrypt content with model's public key, and call Responses API with the user's public key in header;
4. Decrypt the received chunk with user's private key.

Remaining issues:

• For loading the conversations, some changes will be required in cloud-api for storing the conversations. Now all the encrypted chunks are concatenated with no easy way to distinguish each chunk, making it hard to decrypt the messages by the user.
• Another solution is decrypting each chunk at frontend, encrypting the concatenated message and inserting the whole message (conversation item) back into the conversation.