fix: preserve Workchat document selection

[WangShen007]

Summary

• Fixes 学习台文档文件在切换导航后没有保持选中状态 #4
• Preserve the selected Workchat document context while switching away from and back to the Workchat page
• Move the Workchat context item state from a component-local ref into a small session-scoped Pinia store

Rationale

The selected document context was stored inside src/views/workchat/index.vue as a local ref. When navigating to another left-side page, the Workchat component can be unmounted, so the local state is recreated when returning and the selected document is no longer kept.

This PR keeps the change intentionally small and session-scoped:

• no localStorage/sessionStorage persistence
• no route-level behavior change
• no database or config changes
• state still resets after app reload

Testing

Tested on Windows 11:

• pnpm exec vite build passed
• launched the app with pnpm dev
• confirmed the Reviva Electron window opened
• manually verified the document selection remains after navigating away from Workchat and then returning