fix: add stream_count view fn and fix resume_stream status

[Anthony-19]

Closes #421

• Add stream_count() read-only function that returns the global StreamCounter value (monotonically increasing, equals the highest stream ID ever issued, never decrements on cancel/complete).
• Returns 0 on a freshly-deployed contract with no streams.
• Add TypeScript binding fetchStreamCount() in frontend/src/lib/soroban.ts that simulates the view call without wallet auth.
• Unit tests: stream_count_returns_zero_on_fresh_contract, stream_count_increments_by_one_per_create, stream_count_is_not_decremented_by_cancel, stream_count_matches_last_stream_id.

Closes #787

• resume_stream now checks stream.status == Paused (not just stream.paused), so a stream cancelled while paused (status=Cancelled, paused=true) correctly returns StreamInactive instead of being resurrected to Active.
• cancel_stream now explicitly clears stream.paused = false and stream.paused_at = None when settling a cancellation, ensuring no stale pause state survives into the Cancelled record.
• Unit tests: resume_after_cancel_while_paused_returns_stream_inactive, cancel_while_paused_clears_pause_fields, cancel_normal_stream_also_clears_pause_fields.