feat(mindspace): 悬浮聊天勾选资料联动、UI 与交互优化

勾选单个文件时 Agent 获知上下文并主动问候；打开悬浮窗使用新会话。
统一弹窗浅色主题、固定高度与输入区布局，支持点击外部收起。

Co-authored-by: Cursor <cursoragent@cursor.com>

src/utils/mindspaceChatContext.ts

@@ -11,6 +11,7 @@ export type { MindSpaceChatContext } from '../types';
 export {
   buildContextPrefix,
   buildMindSpaceChatContext,
+  buildSelectedAssetGreeting,
   formatContextChip,
 } from '../../mindspace-chat-context.mjs';
 
@@ -27,6 +28,7 @@ export type BuildMindSpaceChatContextInput = {
     content: string;
   } | null;
   assets?: MindSpaceAsset[];
+  selectedAssets?: MindSpaceAsset[];
   focusedAsset?: MindSpaceAsset | null;
   route: string;
   agentSessionId?: string | null;