fix: convert last_performed_odometer to km before storing on maintenance schedules

[andrew-demers]

Summary

Maintenance schedules stored last_performed_odometer as a raw form value without unit conversion, while MaintenanceSchedule.calculate_next_due() assumes km-based storage (it adds interval_miles * 1.60934 directly to last_performed_odometer). For vehicles set to miles, this meant the interval was converted to km and added to a value that was still in miles, producing a wildly incorrect next_due_odometer.

Example: a vehicle at 29,711 mi with a 5,000 mi service interval should show the next service due at 34,711 mi. It previously showed ~37,758 mi.

Changelog

• Fixed: last_performed_odometer is now converted to km (the app's internal odometer convention) before being stored, based on the vehicle's effective odometer unit — in new(), edit(), and complete() in app/routes/maintenance.py.
• Fixed: next_due_odometer is now converted back to the vehicle's display unit wherever it's rendered (maintenance/index.html, vehicles/view.html), instead of showing the raw km value labeled with the wrong unit.
• Fixed: the maintenance edit form now converts the stored km value back to the vehicle's display unit when pre-filling the odometer field, and shows the unit in the field label.

Testing

• Tested locally
• Tested with Docker image

Verified with the reported repro case: a mi-unit vehicle with last_performed_odometer = 29,711 mi and a 5,000 mi interval now correctly shows the next service due at 34,711 mi.