[M1][Epic 1.4] Build React dashboard with login, source management, and analytics

[POWDER-RANGER]

Problem

Create minimal frontend allowing users to: login → add RSS source → trigger ingestion → view sentiment summary.

Tasks

• Implement login screen (email/password form)
• Store JWT in localStorage/sessionStorage
• Create sources list page
• Add "Add RSS Source" form with URL validation
• Add "Run Ingestion" button for each source
• Build dashboard with cards:
  • Total documents ingested
  • Average sentiment score
  • Recent alerts list (last 10)
• Add loading states for async operations
• Add empty states for each section
• Implement logout functionality

Acceptance Criteria

• ✅ Non-technical user can: login → add RSS → run → see sentiment summary
• ✅ No broken navigation links
• ✅ Empty states display when no data exists
• ✅ Error messages shown for failed operations

Priority: P0 - Blocker
Labels: frontend, react, M1, P0