bugfix: match knowledge library font weight to the sidebar

[Grace-dev11]

Summary

• Knowledge library text was heavier than the app sidebar (weights 600–800 on titles, lists, and buttons).
• The library now uses the same sans family and regular weight as the sidebar. Section titles use medium weight only.

Test plan

• Open 知识库 and compare the library list and main panel with the leftmost app sidebar. Body text, library names, and buttons should match the sidebar weight.
• Section labels such as 知识库 and 个人知识库 stay only slightly stronger than body text.
• Upload, empty state, and dialogs use the same weights.

Made with Cursor

[Grace-dev11]

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