Fix locale-safe UI scale input

[Loggableim]

Summary

• accept a decimal comma for numeric slider input without changing expressions
• validate UI scale at config load, application, import, and save boundaries
• add focused regression checks

Root cause

The editable UI Scale slider evaluated 1,5 as an expression rather than a decimal number, which could persist an invalid scale and prevent a later startup.

Validation

• python paint/tests/test_issue_2082.py
• Local native project generation is currently blocked by the bundled amake stack-overflow error; GitHub platform builds will provide compile validation.

Fixes #2082