[v2] implement collaborative (coinjoin) send flow in send page #1137
Description
in v2 send page, collaborative (coinjoin) mode exists in ui but the flow is not fully wired end-to-end. we need to implement the full collaborative send path so users can run coinjoin sends from send page with clear status and feedback.
scope:
wire collaborative send start/stop to jm api in send flow
show lifecycle states: starting, running, completed/ended, failed
support abort flow with clear feedback
add/validate collaborator count input in send options
show precondition warnings (confirmations/retries) before send
show collaborator fee estimate in send/confirm flow
auto-refresh wallet/jar balances during and right after flow
@theborakompanioni