Additional field specs and customizable dimensions

Follow-up to the MVP (#1), which ships with a single hard-coded regulation Ultimate field.

## Feature

Support additional field specifications:

- **Preset variants**: e.g. a smaller field for casual play, youth dimensions.
- **Custom dimensions**: let the user adjust individual measurements — end zone depth, central zone length, field width — and save named custom specs.
- **Unit support**: display/edit dimensions in yards or meters.

## Notes

- The MVP models a field spec as data (width, central zone length, end zone depth), so this is largely UI (a spec picker + editor) plus persistence for saved specs.

Start by reviewing the MVP's field-spec data model and the existing field UI described in issue #1. Trace how the current hard-coded Ultimate field is selected and displayed, then scope the picker, dimension editor, unit conversion, and persistence for named specs. Done means preset and custom dimensions can be selected, edited in yards or meters, and saved.

Written by the indexing model from the issue text.