Constrain macOS overscroll to conversations

[thomaspblock]

Summary

• add a root webview wheel-boundary lock so non-conversation chrome cannot chain wheel gestures to the WKWebView viewport and rubber-band the whole app
• lock html, body, and #root to the viewport with hidden overflow / no overscroll behavior
• mark the real conversation scrollers (main timeline and thread panel), keeping any allowed elastic affordance scoped to conversations
• add a smoke E2E test covering top chrome, sidebar pinned/nav area, channel header, and conversation timeline behavior

Verification

• corepack pnpm --dir desktop typecheck
• corepack pnpm --dir desktop lint
• corepack pnpm --dir desktop exec playwright test tests/e2e/overscroll-boundary.spec.ts --project=smoke
• corepack pnpm --dir desktop build

Verification video: https://sprout-oss.stage.blox.sqprod.co/media/5f9ff3dfdecea8b6e04d27146e9ea8b4b5c1925a04ec6019a53264373c4a2019.mp4

Draft while Thomas manually verifies the macOS trackpad/mouse feel in the staging dev app.