[Fix] Replace per-token forced reflow with ResizeObserver and fix Virtuoso dual scroll context

[samzong]

Summary

ChatView had two scroll architecture flaws: (1) scrollToBottom fired on every streaming token via useEffect on activeTurn?.streamingText, causing synchronous scrollTop = scrollHeight forced reflow ~30 times/second; (2) Virtuoso nested inside overflow-y-auto created competing scroll contexts, with StreamingMessage rendered outside Virtuoso's scroll ownership.

Type of change

• [Fix] bug fix

Why is this needed?

Per-token forced reflow causes visible jank on mobile during streaming. The dual scroll context means streaming content may be clipped or unreachable when virtualization is active (≥100 messages).

What changed?

• Replaced scrollToBottom callback + useEffect depending on activeTurn?.streamingText with a single ResizeObserver on a content wrapper div — zero coupling to React data flow, fires at most once per frame
• Extracted tail content (StreamingMessage, thinking indicator, connecting status) into ChatTail component whose prop shape { context?: TailContext } directly matches Virtuoso's Footer API
• Non-virtualized path: overflow-y-auto + scrollRef + contentRef wrapper observed by ResizeObserver
• Virtualized path: overflow-hidden, no scrollRef; tail content rendered via Virtuoso components.Footer so followOutput accounts for streaming height changes
• VIRTUOSO_COMPONENTS defined at module level for stable component identity (prevents unmount/remount per render)

Architecture impact

• Owning layer: renderer (PWA)
• Cross-layer impact: none
• Invariants touched from docs/architecture-invariants.md: none
• Why those invariants remain protected: change is purely renderer-side scroll behavior, no DB/IPC/protocol changes

Linked issues

Closes #

Validation

• pnpm lint
• pnpm test
• pnpm check:ui-contract
• pnpm check (full suite: lint + architecture + ui-contract + renderer-copy + i18n + dead-code + format + typecheck + test)
• Manual smoke test

Commands, screenshots, or notes:

pnpm check — all gates passed (92+59+6 tests, zero failures)

Screenshots or recordings

No visual change. Scroll behavior is functionally identical — the fix eliminates forced reflow during streaming and resolves scroll context conflicts in virtualized mode.

Release note

• No user-facing change. Release note is NONE.

NONE

Checklist

• The PR title uses at least one approved prefix: [Feat], [Fix], [UI], [Docs], [Refactor], [Build], or [Chore]
• The summary explains both what changed and why
• Validation reflects the commands actually run for this PR
• Architecture impact is described and references any touched invariants
• Cross-layer changes are explicitly justified
• The release note block is accurate

[github-actions]

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