fix(backlight): clamp scroll step to min-brightness

[gitmpr]

What does this PR do?

When min-brightness is set, a scroll step that would cross it wasn't being clamped. Only scrolling while already at or below the minimum was blocked. With scroll-step: 5 and min-brightness: 1, scrolling from 5% passes the guard (5 > 1) and sets brightness to 0%.

The check in handleScroll in src/modules/backlight.cpp was:

if (backend.get_scaled_brightness(preferred_device_) <= min_brightness &&
    ct == util::ChangeType::Decrease) {
  return true;
}
backend.set_brightness(preferred_device_, ct, step);

This blocks further decrease once you're already at the minimum, but doesn't prevent a step from landing below it.

The fix computes whether the post-step value would fall below min-brightness and, if so, sets exactly min-brightness instead of applying the full step.

Why does this matter?

On intel_backlight (type raw, no kernel-enforced minimum), the kernel and logind both accept brightness 0, which on most laptop panels is a completely black screen. This makes the issue a real usability problem rather than just a cosmetic one. Users setting min-brightness: 1 to prevent this are silently unprotected when scrolling in steps larger than 1.

Tested on intel_backlight, max_brightness 7500. Confirmed bug on unpatched master, confirmed fix on this branch.

Relates to #5209 (which adds min-brightness to the default config, which is complementary, but without this fix the protection has gaps whenever scroll-step > 1).