core: build sponsor portfolio analytics dashboard

[EmeditWeb]

Problem

Sponsors have no analytics view. They cannot see
their yield over time or risk exposure.

What To Build

Build /sponsors/analytics page with:

1. Yield over time line chart
2. Funded loans breakdown pie chart
3. Repayment rate bar chart
4. Risk metrics cards
5. CSV export

Files To Touch

• src/pages/SponsorAnalytics.tsx (new)
• src/router/index.tsx
• src/services/sponsors.service.ts

Acceptance Criteria

• All 4 charts render correctly
• Date range selector works
• CSV export works
• Charts responsive on all screen sizes
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue