Add watchlist bounty pool chart

[nikhilsinghal005]

Summary

• add the missing Watchlist -> Bounties chart toggle to the existing watchlist options panel
• render a Bounty Pool by Repository chart for the filtered watched bounty set
• add a tested helper for aggregating watched bounty pool totals by repository

Related Issues

Fixes #1018

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other (describe below)

Screenshots

Before - Watchlist Bounties filters, no chart control:

After - Filters include Chart control:

After - Bounty Pool by Repository chart shown:

Note: Screenshots were captured locally with Playwright-mocked bounty rows because the test API blocks localhost via CORS.

Testing

• npm run test -- src/tests/bountyPoolByRepository.test.ts
• npm run lint
• npm run build
• Local Playwright browser check against /watchlist?tab=bounties with mocked watched bounty rows: Chart control appears and renders a visible ECharts canvas

Checklist

• New components are modularized/separated where sensible
• Uses predefined theme (e.g. no hardcoded colors)
• Responsive/mobile checked
• Tested against the test API
• npm run format and npm run lint:fix have been run
• npm run build passes
• Screenshots included for any UI/visual changes

[anderdc]

Duplicate of #1169 which is better scoped/implemented. Closing.