docs: update README for latest features

[Attacker687]

Summary

• update the feature overview for document preview, QA history, and mind maps
• document current mind map APIs and document operations
• align the technology stack and architecture diagram with the implementation
• refresh testing and troubleshooting guidance

Verification

• backend: 46 tests passed
• frontend: npm ci and npm run build passed (existing chunk-size warnings remain)

[Copilot]

Pull request overview

This PR refreshes the project’s README to reflect recently implemented product capabilities (document preview, QA history, mind maps), current API surface, and updated architecture/testing/troubleshooting guidance.

Changes:

• Expanded the “功能概览” and “使用流程” sections to include document library/preview, QA history viewing, and mind map generation & interaction flows.
• Updated the API table to document current document operations and mind map endpoints.
• Adjusted the tech stack and architecture diagram to match the current frontend dependencies and backend responsibilities, and refreshed test/troubleshooting notes.

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