feat: add weekly financial digest

[xinku1234]

Summary

• Add weekly financial digest data helpers with computed totals, risk breakdowns, AI insights, and recommended actions.
• Add a /weekly-digest dashboard page with summary cards, portfolio distribution, daily trend, risk table, holdings, and AI recommendations.
• Add navigation entry, README documentation, and focused tests for digest calculations.

Test Plan

• npm test -- --runInBand src/__tests__/weeklyDigest.test.ts
• npm test -- --runInBand
• npm run build

Closes #121

/claim #121

[xinku1234]

Demo evidence update:

• Added /weekly-digest route for the Weekly Financial Digest UI.
• Verified the Vite local preview is reachable at http://127.0.0.1:5173/.
• Opened the route locally and confirmed the page renders with the weekly digest cards/sections.
• Ran the focused frontend verification successfully.

This PR body already includes /claim #121 for the bounty claim.