fix(webview): restore conversation virtualization

[AprilNEA]

Summary

• constrain the Web workbench grid child so the conversation keeps a real scroll viewport
• give virtua a 300px initial turn-size hint to avoid an expensive first measurement
• add a bundled browser regression check for a scrollable, bottom-pinned, virtualized long thread

Root cause

The direct grid child in WebWorkbenchShell lacked min-h-0. Its automatic minimum size expanded to the transcript's min-content height, so virtua saw the entire 48-turn thread as visible and mounted every row during a thread switch. The earlier scroll-to-bottom change was not the cause.

With the constrained height chain restored, the 48-turn mock mounts 4 rows instead of 48. The initial virtualizer layout callback dropped from roughly 150–170ms to 37ms; five short/long round trips painted in 11–16ms with no long tasks.

Validation

• pnpm -F @linkcode/webview e2e:browser
• pnpm check:ci
• pnpm test — 302 files and 2435 tests passed

Linear: CODE-259

[linear-code]

CODE-259