Add Admin Sweep Function for Unclaimed $cDCU

[LuminaEnvision]

Some leftover balances should be sweepable by the admin/community wallet for treasury control.
Expected work: Add a sweep function in DCURewardManager. Ensure user balances remain untouched.
Why this matters: Allows long-term maintenance and treasury control.

[BeRegen]

Frontend status update.

The frontend does not expose or assume any admin sweep functionality.
Unclaimed rewards are treated as contract-owned state.

An admin sweep function can be added entirely at the contract level
without impacting the UI.

Issue remains contract-scoped.