[Contract] Streaming / time-based payments

[Gbangbolaoluwagbemiga]

User story

For ongoing/retainer work, clients want continuous (streaming) payment — funds unlock to the
freelancer linearly over time rather than only at discrete milestones (salary-style streaming).

What needs to be done

• New escrow mode: define a stream with start, end, and rate (amount per ledger/second)
• withdrawable(escrow_id) view computing the vested-but-unclaimed amount at now
• claim_stream(escrow_id, beneficiary) transferring the vested amount; client can cancel_stream
  to reclaim the unvested remainder
• Guard against rounding/overflow; reuse the pause ([Contract] Global circuit-breaker / emergency pause across all mutating entrypoints #30) and event ([Contract] Emit structured events for all state transitions #5) infrastructure
• Frontend: a live "streaming" progress UI

Acceptance criteria

• Vested amount computed correctly across the timeline (unit-tested at boundaries)
• Freelancer can only claim vested funds; client can only reclaim unvested funds
• No path lets total claimed + reclaimed exceed the deposit (ties to [Contract] Fund-conservation invariant + property tests #28)

Relevant files

new module under contracts/secureflow/src/, lib.rs