fix(code): preserve session scroll position across window focus

[sakce]

Summary

• The conversation view's visibilitychange handler in ConversationView.tsx was calling scrollToBottom() unconditionally whenever the document became visible — so cmd+tab away and back always reset the user's scroll position.
• Mirror the at-bottom state already reported by VirtualizedList into a ref and only re-pin to the bottom on visibility return when the user was already following the stream. Manual scroll positions are now preserved across focus changes; streaming auto-scroll continues to work because it has its own at-bottom check.

Test plan

• pnpm --filter code typecheck
• pnpm --filter code test (947 pass; one unrelated archive/service.integration.test.ts > unarchive with recreateBranch timeout in main-process git code)
• Manual: scroll up mid-conversation, cmd+tab away and back → position preserved, scroll-to-bottom button stays visible
• Manual: scroll to bottom, cmd+tab away and back → still pinned to bottom
• Manual: while at bottom with a streaming response, hide the window and return → newest content visible

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(code): preserve session scroll posit..." | Re-trigger Greptile}