Allow fuel expenses to link directly to trucks/drivers without forcing trip-level assignment, and fix per-truck per-mile reports

As an active motor carrier running commercial units, accurate per-mile reporting and driver settlement deductions are essential, but the current system creates major roadblocks:

1. Decouple Fuel from Trips: Right now, fuel expenses must be tied to a specific trip to show up as a deduction on a driver's pay settlement. In reality, fuel carries over across multiple loads in the tanks, making trip-level assignment pure guesswork. We need the ability to assign fuel purchases directly to a truck or driver independently so deductions flow onto settlements automatically.

2. Per-Truck Per-Mile Reports with Driver Pay: Current reports force us to choose between per-mile stats that ignore driver pay (Company Overview/Unit Revenue) or profit metrics that lack per-mile breakdowns (Unit Operating Income). We need native, automated per-truck cost-per-mile and profit-per-mile reporting that includes driver payroll.

If you run company trucks and want to stop doing manual cross-report math and forced trip fuel entry, please upvote this so the development team prioritizes it.