Expense form should auto-select logged-in user

Description

When adding an expense, the form requires manually selecting a user from a dropdown. With Firebase authentication now providing the logged-in user's identity, the form should auto-populate with the current user.

Current Behavior

• User must manually select themselves from dropdown when creating an expense

Expected Behavior

• req.dbUser (populated by auth middleware) should be used to auto-select the current user
• User dropdown could be hidden or shown as read-only for regular users
• Admins might still need ability to create expenses on behalf of others

Context

• Firebase auth now provides req.dbUser.id on all authenticated requests
• This is a minor UX improvement, not blocking

Files Likely Involved

• frontend/src/pages/Expenses.tsx or similar expense form component
• May need to pass user context from auth to the form