fix(preview): restore nested panel scrolling

[Yuqing (mydmdm)]

Summary

• restore native vertical scroll chaining from truncated and horizontal-overflow descendants to the outer Preview/Chat scroller
• retain overscroll containment at the side-panel boundary so exhausted scrolling does not reach Canvas
• document the panel scroll-ownership contract and add a real-browser regression

Fixes #259.
Related to #251.

Scope

This is the independently verified Chat/panel portion discovered during #251. It does not change the separately investigated PDF listener timing, intermittent Canvas paste behavior, or documented Canvas context-menu suppression; the PDF-specific report remains open in #251.

Validation

• pnpm --dir apps/web exec playwright test --config playwright.overlay-panels.config.ts --grep "panel descendants chain"
• full overlay-panel browser suite: new regression and 7 existing cases passed; the known unrelated narrow-layout toolbar-centering case remains failing
• pnpm typecheck
• pnpm format
• pnpm lint:fix (0 errors; existing warnings only)