refactor(web): remove virtual list polling

[xingkaixin]

Summary

• replace the virtual message list's permanent 100 ms layout polling with event-driven updates
• rebind scroll and resize listeners when the effective scroll container changes
• react to container geometry, ancestor layout attributes, and font loading events
• add regression coverage for idle behavior, container resize, and asynchronous row growth

Why

Every virtualized detail view performed geometry reads ten times per second even when the page was idle. Existing observers covered row growth, but the viewport lifecycle still depended on polling for container and layout changes.

The updated lifecycle coalesces relevant events with requestAnimationFrame and performs no recurring work while the page is stable.

Validation

• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test (650 passed)
• pnpm test:e2e (6 passed)

Closes CS-42