Adds a live cost ticker (⚡ ~N sats) visible in the top-right HUD
during active paid interactions. The ticker appears when a work
invoice is issued with the estimated cost, updates to the final
charged amount when the job completes, and auto-hides after 5s.
Changes:
- event-bus.ts: add CostEvent type { cost:update, jobId, sats, phase, isFinal }
- events.ts: translate cost:update bus events → cost_update WS messages
- jobs.ts: emit cost:update with estimated sats on invoice creation,
and again with actual sats when work completes (paid jobs only)
- sessions.ts: emit cost:update with debitedSats after each session request
- ui.js: add showCostTicker/updateCostTicker/hideCostTicker (fixed-position HUD badge)
- websocket.js: handle cost_update messages, drive the cost ticker
Fixes#68
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
