Refactor PDA derivation for User account creation to avoid index-based collisions

[juan-malbeclabs]

Currently, when creating a User account, the PDA derivation still relies on an incremental index. This approach introduces a potential race condition when multiple users attempt to connect at the same time, since they may try to claim the same index.

Proposed Change

• Update the PDA derivation method for User accounts so it no longer depends on the index.
• Ensure uniqueness and determinism by using a combination of stable seeds (e.g., user pubkey, device identifier, or other unique attributes).
• Validate that the new PDA derivation remains compatible with existing flows and does not break user discovery or account lookups.

Impact

• Eliminates race conditions during concurrent user creation.
• Improves reliability and scalability of the User account provisioning flow.
• Reduces the risk of account conflicts or unintended overwrites.

Next Steps

• Define the new PDA seed strategy.
• Implement and update the relevant create_user instruction.
• Adjust unit tests and e2e tests to cover concurrent account creation scenarios.