Add multi-account dashboard overview

[darshan-Jahagirdar]

Summary

• Add account records with create/list/update API support
• Allow transactions to be associated with accounts
• Extend /dashboard/summary with active accounts, account count, and total account balance
• Show account balances in the dashboard UI
• Add focused backend tests and README documentation

Closes #132

Validation

• From packages/backend: .\.venv-backend-test\Scripts\python.exe -m pytest tests\test_accounts.py -q -> 2 passed
• From packages/backend: .\.venv-backend-test\Scripts\python.exe -m black --check app\models.py app\__init__.py app\routes\accounts.py app\routes\__init__.py app\routes\expenses.py app\routes\dashboard.py tests\test_accounts.py
• From packages/backend: .\.venv-backend-test\Scripts\python.exe -m flake8 app\models.py app\__init__.py app\routes\accounts.py app\routes\__init__.py app\routes\expenses.py app\routes\dashboard.py tests\test_accounts.py
• python -m py_compile packages\backend\app\__init__.py packages\backend\app\models.py packages\backend\app\routes\__init__.py packages\backend\app\routes\accounts.py packages\backend\app\routes\dashboard.py packages\backend\app\routes\expenses.py packages\backend\tests\test_accounts.py
• From app: npm ci && npm run build
• git diff --check

Note: npm ci reported existing dependency audit findings in the frontend dependency tree; the production build still completed successfully.

/claim #132

[darshan-Jahagirdar]

Validation update for this PR:

Frontend:

cd app
npm test
npm run build

Result: 9 Jest suites / 27 tests passed, and the production Vite build completed successfully.

Backend:

python -m compileall -q packages\backend\app
git diff --check

Result: both passed locally.

I also ran the branch-specific backend tests in a temporary Python 3.11 virtualenv with packages/backend/requirements.txt installed:

cd packages/backend
python -m pytest tests\test_accounts.py

Result: 2 tests passed.

Full backend pytest is blocked in this local environment because auth-dependent tests try to connect to Redis at redis:6379, which is not resolvable here.