Fix unicode fraction conversion

Description
When using unicode fractions, the web UI reports the ingredient to not be parsable but in fact it is parsable and correctly calculated.

Reproduction
Steps to reproduce the behavior:

1. Create or edit a recipe
2. Add an ingredient with a unicode fraction, e.g. ¾
3. Save and view the recipe
4. Change the recipe yield to trigger recalculation

Expected behavior
No warning is shown to the user

Actual behavior
A warning sign is shown to the user indicating a problem with the calculation (see screenshots).

Screenshots

After changing the yield amount:

Browser
Firefox

Versions
Nextcloud server version: 30
Cookbook version: 0.11.1
Database system: MySQL

Maybe @Weissnix4711 or @j0hannesr0th, can you have a look at this? Ideally, I would say that the same Regex is used to check and to do the actual calculation to avoid such conflicts. That would mean some restructuring but should be managable.

Start by tracing the ingredient parsing, warning display, and recalculation paths described in the reproduction, then compare how unicode fractions are validated and calculated. Reproduce the issue with ¾ and changing the recipe yield. Done means the valid ingredient recalculates without showing a warning.