feat(web): add Obsidian export panel to Research workspace

[jerry609]

Parent Epic

#159 — [Epic] Obsidian CLI 集成 — 研究知识管理

Summary

Add an Obsidian export panel to the Research workspace so users can hand off saved papers / active track exports into a local Obsidian workflow without leaving the dashboard.

Why here

The right placement is the Research workspace, not DeepCode Studio:

• export scope is defined by active Track + saved papers
• the Research page already owns paper save/export actions
• Studio is code-generation-centric and does not own library/export state

Proposed UX

• Add an Obsidian action in the Research workspace export area
• Show current export scope: active track or global saved library
• Provide a copyable local CLI command, for example:
  • paperbot export obsidian --track-id <id> --vault "/path/to/vault"
• Provide a markdown fallback download using the existing paper export route
• Explain that filesystem write happens through local CLI, not the hosted dashboard

Acceptance Criteria

• Research workspace shows an Obsidian export entry point near saved-paper export actions
• Panel shows current scope (Track vs Global) and paper count
• Panel can generate/copy a correct paperbot export obsidian ... command
• Panel links to or triggers markdown fallback export for the same scope
• Empty-state UX is clear when there are no saved papers
• UI copy makes it explicit that vault writes are local CLI operations
• Component has unit coverage and browser verification