fix(desktop): stabilize members dialog scrolling

[matt2e]

Summary

• Keep virtualized member rows measurable by removing content-visibility: auto from the measured row subtree.
• Use the member card's 60px baseline as the virtualizer estimate while retaining deferred rendering for eager search and archived-member lists.
• Cover large rosters with a regression test that checks stable scroll extent across the list and verifies the final member remains reachable.

Related issue

None found.

Testing

• just ci
• pnpm -C desktop build:e2e
• pnpm -C desktop exec playwright test tests/e2e/channels.spec.ts --grep 'members sidebar virtualizes large channel rosters' --repeat-each=5

Before

members-before.mov

After

members-after.mov