Selected unit should only affect display, not saved data. Data should be stored in standardized, convertable size instead.

At the moment, Open Library has different units for saving the measurements of a book: inches and cm for size, g and ounces for weight.

Those measurements are stored directly into the book record, so if I enter a value as "inches", I will only ever get the book's size in inches. This is inconvenient for users who use another measurement system.

Instead of hard storing a specific size units, all measures should be stored in standardized manner (i. e. micrometer for size, microgram for weight) and converted only for display purposes. So, a 6 inch / 15.24 cm book would be stored as 152400 micrometer and could be displayed as either (or any other) format, depending on user choice. This would also make it easier to integrate non-western measurements if those ever become necessary.

### Describe the problem that you'd like solved
Weight and size units should be chosen by the user, not the database entry.

### Proposal & Constraints

Many old forum software from the 2000s had an entry where I could choose time format, starting day of the week, formal / informal addressing. I assume something similar could be implemented for Open Library.

The selected unit for storage needs to be small enough to convert to most common measurement units.

### Additional context
#2587
#2242

### Stakeholders