Fix locale-safe UI scale input - #2099

Good fix — decimal-comma locales breaking the UI scale input is a real papercut. The normalization helper is clear and the window-scale validation (clamping to 1.0–4.0) is a sensible guard against a corrupt config.

Two things:

1. ui_normalize_numeric_text uses a static char normalized[1024] buffer. Static buffers in UI code are a re-entrancy hazard if this ever gets called from two places in the same frame. It's probably fine today (single-threaded UI), but a caller-provided buffer or a comment would make it safer.

2. The config_validate_window_scale check if (!(scale >= 1.0 && scale <= 4.0)) — the !(scale >= 1.0) form also catches NaN, which is actually a nice touch. Just confirming that's intentional (it reads a bit unusual).

Otherwise LGTM.