[3/6] Consolidate chat and topic actions / 统一会话与轮次操作入口

[SivanCola]

Summary / 摘要

• Move copy, branch, summarize, and rewind actions into the bottom action row for each assistant turn.
• Keep topic-level menus focused while making per-turn operations easier to discover.
• Preserve action behavior while adapting it to the updated chat layout.

Validation / 验证

• npm run typecheck && npm run build
• go test . -run 'TestPreviewSessionMessages|TestHistoryMessages|TestToWireTool' (desktop)\n- go test ./internal/event ./internal/serve ./internal/agent ./internal/control\n- git diff --check

[chatgpt-codex-connector]

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