[MN4][P0] Add connection diagnostics and export preview

[evan188199-tech]

Tracked by #10. Local plan: docs/roadmap/marginnote-4.md.

Goal (中文): 连接前先检查 MN4 导出目录：展示兼容状态、文件/高亮/笔记/脑图统计和解析警告；只有确认可解析后才允许连接。

Do you need to file a feature request?

• I have searched the existing feature requests and this feature request is not already filed.
• I believe this is a legitimate feature request, not just a question or bug.

Feature Request Description

Add a connection-time probe so users can validate a MarginNote 4 export folder before it becomes a Connected KB.

Today POST /api/v1/knowledge/connect-marginnote registers a pointer with little validation, and CreateKbModal does not probe MN4 folders the way linked RAG folders or LightRAG servers are probed.

Add POST /api/v1/knowledge/probe-marginnote that:

• does not register a KB
• does not write into the export folder
• still goes through the existing path access policy
• returns compatibility status, file counts, document/highlight/note/mind-map counts, parse warnings, and writeback-directory availability

The Create KB modal should gain a Check folder action, loading/error/empty states, and a stats preview. Connect stays disabled until the probe finds parsable content.

Related Module

Knowledge Base Management

Use Case

A user pastes an export path, clicks Check folder, and immediately sees whether DeepTutor can read the notebook. An empty, damaged, or inaccessible folder fails before a dead KB is created.

Additional Context

Priority: P0
Phase: M1
Status: Planned
Depends on: parser hardening
Blocks: dedicated notebook detail experience

Key files

• deeptutor/api/routers/knowledge.py
• deeptutor/capabilities/marginnote/data/base.py
• web/lib/knowledge-api.ts
• web/components/knowledge/CreateKbModal.tsx
• web/locales/en/app.json
• web/locales/zh/app.json

Acceptance criteria

• POST /api/v1/knowledge/probe-marginnote returns compatibility, counts, warnings, and writeback availability without registering a KB or writing files.
• Path access policy is enforced; forbidden or remote-inaccessible local paths fail clearly.
• Create KB modal has Check folder, loading, error, empty, and stats-preview states.
• Connect is enabled only after the probe finds parsable content.
• Empty folder, corrupted files, permission errors, and no-content folders are recoverable UI states.

Success metric
Users can reject a bad export before connecting and only create KBs that DeepTutor can actually read.