Fix technical debt: resolve N+1 query and unnecessary re-renders

[shawnSpy]

What I Did

Fixed two performance issues:

1. Backend: Resolved N+1 query problem in transaction list endpoint by adding eager loading of category relationships using leftJoin and addSelect.
2. Frontend: Prevented unnecessary re-renders of TransactionList by wrapping it with React.memo() and using useCallback() for the user profile click handler.

Depends On

N/A

How to Test

1. Start the application: ./run.sh
2. Navigate to http://localhost:3000
3. Click the "User Profile" button multiple times
4. Verify that the transaction list does not flicker or re-render unnecessarily
5. Confirm transactions and categories still display correctly
6. Run test suite: ./run-tests.sh to ensure existing functionality works

Architectural Decision Record (ADR)

• Backend: Used Doctrine's leftJoin with addSelect for eager loading, matching the pattern already used in show() and delete() methods. This ensures all category data is fetched in a single query instead of N+1 queries.
• Frontend: Applied React.memo() to prevent re-renders when props haven't changed, and useCallback() to maintain stable function references. This is a lightweight solution that doesn't require restructuring the component hierarchy.

AI Usage Summary

Used AI tools to identify the root causes of both performance issues and verify the fixes follow best practices.