feat(desktop): DeerFlow visual redesign + stream hang fix + intelligence client

DeerFlow frontend visual overhaul:
- Card-style input box (white rounded card, textarea top, actions bottom)
- Dropdown mode selector (闪速/思考/Pro/Ultra with icons+descriptions)
- Colored quick-action chips (小惊喜/写作/研究/收集/学习)
- Minimal top bar (title + token count + export)
- Warm gray color system (#faf9f6 bg, #f5f4f1 sidebar, #e8e6e1 border)
- DeerFlow-style sidebar (新对话/对话/智能体 nav)
- Reasoning block, tool call chain, task progress visualization
- Streaming text, model selector, suggestion chips components
- Resizable artifact panel with drag handle
- Virtualized message list for 100+ messages

Bug fixes:
- Stream hang: GatewayClient onclose code 1000 now calls onComplete
- WebView2 textarea border: CSS !important override for UA styles
- Gateway stream event handling (response/phase/tool_call types)

Intelligence client:
- Unified client with fallback drivers (compactor/heartbeat/identity/memory/reflection)
- Gateway API types and type conversions

desktop/src/components/ai/index.ts

@@ -0,0 +1,11 @@
+export { Conversation, ConversationProvider, useConversationContext } from './Conversation';
+export { ReasoningBlock } from './ReasoningBlock';
+export { StreamingText } from './StreamingText';
+export { ChatMode, type ChatModeType, type ChatModeConfig, CHAT_MODES } from './ChatMode';
+export { ModelSelector } from './ModelSelector';
+export { TaskProgress, type Subtask, TaskProvider, useTaskContext } from './TaskProgress';
+export { SuggestionChips } from './SuggestionChips';
+export { ResizableChatLayout } from './ResizableChatLayout';
+export { ToolCallChain, type ToolCallStep } from './ToolCallChain';
+export { ArtifactPanel, type ArtifactFile } from './ArtifactPanel';
+export { TokenMeter } from './TokenMeter';