feat: web UI doc upload — drag-and-drop in viewer #23

Description

Add a document upload interface to the viewer. Users drag-and-drop files in the browser, the API stores them and triggers extraction. This is the bridge between CLI-only and the non-technical user path.

Acceptance Criteria

• Upload page in viewer with drag-and-drop zone
• Accepts: .md, .txt, .pdf, .docx files
• Shows upload progress and extraction status
• API endpoint POST /api/documents handles file storage
• Uploaded docs tracked in document store with metadata (hash, filename, timestamp)
• After extraction completes, user redirected to Explorer to see new entities

Out of Scope

• Batch upload of entire folder structures
• Document versioning UI