feat: add balance management

[romy651]

Summary

LibreChat administrators currently have no community admin-panel view for finding users who need more credits or resetting a user's balance limit. This PR adds a Balance section that lists all users, prioritizes pending top-up requests, shows the optional request reason and request age, and provides a validated Reset limit action.

The panel uses the new capability-gated balance API introduced by LibreChat-AI/LibreChat#16774. It keeps request and all-user views independently paginated, reflects pending-request counts, and generates an idempotency key for every administrative credit operation.

Depends on LibreChat-AI/LibreChat#16774.

Change Type

• New feature (non-breaking change which adds functionality)

Test Configuration:

• LibreChat balance feature enabled
• Administrative user with READ_BALANCES and MANAGE_BALANCES
• Tested against the companion LibreChat API branch

Screenshots / recordings

TODO before marking ready: attach light- and dark-mode captures of the Balance list, pending-request view, and Reset limit dialog. This PR is intentionally being opened as a draft while those assets are prepared.

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in complex areas of my code
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective
• Local unit tests pass with my changes
• The dependent LibreChat API change has been merged and published