Fix tall chat transcript scrolling

[MaggieAppleton]

Summary

• keep short Chat transcripts bottom-aligned without negative flex overflow
• make older messages reachable when a transcript exceeds the pane
• cover desktop and compact/mobile layouts with a Playwright regression

Root cause

The transcript stack used justify-end. Once its content exceeded the scroll container, flexbox aligned the overflow above the scroll origin, so setting scrollTop to zero still left older messages offscreen. A first-child auto margin preserves bottom alignment only while free space exists and collapses safely for tall content.

Verification

• bun test apps/web/src/chat (51 pass)
• bun run types
• bun run ci
• Playwright: chat keeps the start of a tall transcript reachable across layouts