perf(viewer): coalesce live session refreshes

[benvinegar]

Summary

• coalesce sidebar session-list refreshes across bursts of live post events
• prevent slower session-list responses from overwriting newer sidebar state
• keep a trailing refresh for events arriving during an in-flight or failed request
• add deterministic Chromium coverage for create/update/delete bursts and refresh races

Validation

• npm test
• npm run coverage
• npm run test:worker
• npm run typecheck
• npm run lint
• npm run format:check
• focused Playwright test repeated 20 times on Chromium
• embed SSE and WebSocket Playwright tests

WebKit could not launch locally because required host libraries are unavailable; CI remains the cross-engine check. npm run security:audit currently reports dependency advisories already present on main.

This PR description was generated by Pi using gpt-5.6-sol