refactor(ui): share chat rendering across GUI and WebUI

[su-fen]

Linked issue

Closes #418

Summary

• 将 GUI/WebUI 重复的 AssistantBubble、RoundContent、工具轨迹、工具结果、文件拖放 Overlay、Workspace Overlay、Skill Hook 与排队内容判断迁移到 crates/agent-ui。
• 保留 GUI 的细粒度虚拟化渲染壳；WebUI 直接使用共享完整 AssistantBubble。
• 通过宿主 adapter 保留两端既有平台策略：工具审批、AskUserQuestion、图片延迟加载、运行中折叠内容以及 Workspace Overlay 标题栏与堆叠层级。
• 扩展 UI boundary 检查，阻止宿主重新创建或导入已迁移的本地副本。
• 本次为结构性抽离，不引入用户可见的功能或交互改版。

Change scope

• Modules: agent-ui / agent-gui / agent-gateway/web
• Shared chat rendering: crates/agent-ui/src/components/chat/AssistantBubble.tsx, crates/agent-ui/src/components/chat/assistant-bubble/
• Shared overlays/hooks/helpers: crates/agent-ui/src/components/workspace-editor/WorkspaceOverlayHost.tsx, crates/agent-ui/src/components/chat/FileDropOverlay.tsx, crates/agent-ui/src/lib/
• Host adapters: crates/agent-gui/src/agent-ui-adapters/assistantBubble.ts, crates/agent-gateway/web/src/agent-ui-adapters/assistantBubble.ts
• Boundary enforcement: scripts/check-ui-boundaries.mjs

Screenshots / preview

下图由本分支临时 Vite 预览入口直接渲染共享 AssistantBubble，使用合成数据，不包含本地会话或凭据。抽离前后预期视觉一致。

Verification

• pnpm --dir crates/agent-gui test:frontend — 1501 passed
• pnpm --dir crates/agent-gateway/web test — 540 passed
• pnpm --dir crates/agent-gui build — passed
• pnpm --dir crates/agent-gateway/web build — passed
• pnpm --dir crates/agent-gui exec biome check src/ --diagnostic-level=error --max-diagnostics=50 — passed
• pnpm --dir crates/agent-gateway/web exec biome check src/ --diagnostic-level=error --max-diagnostics=50 — passed
• Changed agent-ui source files checked with Biome — passed
• pnpm check:ui-boundaries — passed
• node --check scripts/check-ui-boundaries.mjs — passed
• git diff --check — passed

Pre-submit checklist

• A requirement issue is linked.
• Synced with the target branch; no merge conflicts.
• The change is focused, with no unrelated modifications.
• No secrets, tokens, or personal data included.
• No user behavior, deployment, or configuration documentation change is required.