The left-column run list was server-rendered once, so runs that started
after page load only appeared on refresh. Add a run-index SSE stream that
re-renders #run-index on run start/stop/cancel, driven by the telemetry
the stats server already collects.

- stats: subscribe_index/0 + notify index subscribers on run
  start/stop/cancelled/budget events
- sse: GET /sse/runs stream (stream_index/1) that subscribes and patches
  #run-index; keeps the open run highlighted via ?active
- page: wrap the list in a stable data-init element around #run-index;
  index/show open the stream