feat(stream): add total_streamed() view method to DripStream

[chainsmt]

Adds total_streamed() as the canonical on-chain view for cumulative streamed tokens, delegating from the existing streamed_total() alias so both symbols stay available.

Also updates the frontend SDK to query total_streamed and adds a unit test asserting parity with streamed_total after elapsed time accrues.

Includes missing Error enum variants (BackdatedStream, StreamUnderfunded, InvalidRecipient) already referenced by lib.rs on main so the contract crate compiles for CI.

Testing

• cargo test total_streamed in contracts/stream → 1 passed

Closes #461

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