feat(dashboard): add multi-account overview with aggregated metrics

[yuliuyi717-ux]

Closes #132

Summary

• add GET /dashboard/multi-account-overview to aggregate monthly income, expenses, net flow, and upcoming bills across selected accounts
• include per-account summary blocks and roll-up totals in the API response
• wire frontend dashboard API + UI to show combined totals and per-account overview
• add backend/Frontend tests and README note for the new endpoint

Validation

• sh ./scripts/test-backend.sh tests/test_dashboard.py::test_multi_account_overview_aggregates_and_returns_per_account
• sh ./scripts/test-backend.sh tests/test_dashboard.py
• cd app && npm test -- --runTestsByPath src/__tests__/Dashboard.integration.test.tsx --testNamePattern "shows multi-account combined totals and per-account overview"
• cd app && npm test -- --runTestsByPath src/__tests__/Dashboard.integration.test.tsx
• cd app && npm run build

[yuliuyi717-ux]

Hi maintainers, just a gentle follow-up on this PR when you have a moment. It adds the multi-account dashboard overview with aggregated metrics, and I’d really appreciate a review whenever it fits your queue. Thanks!