feat: Smart digest with weekly financial summary

[ViVaLaDaniel]

Summary

Implemented the Weekly Financial Summary as requested in issue #121.

Changes

• Backend API: Added weekly_financial_summary in packages/backend/app/services/ai.py that calculates week-over-week trends, aggregates spending, and generates a smart digest via Gemini (or heuristic fallback).
• Routes: Exposed GET /insights/weekly-digest in packages/backend/app/routes/insights.py.
• Frontend Types: Updated �pp/src/api/insights.ts to include WeeklyDigest interface and getWeeklyDigest API client.
• UI: Updated the Analytics dashboard (�pp/src/pages/Analytics.tsx) with a toggle switch to toggle between the original Monthly Budget view and the new Weekly Digest view. The digest presents a short summary, highlighted trends, health score, and an action items checklist.