fix(metadata): recall z_image_shift correctly

[Pfannkuchensack]

Summary

Widen the validator range to match the param's actual max (10) so saved shift values above 3 are recalled, and handle absent/null values by recalling as auto (null) when the recalled image is a Z-Image.

Previously the parser used z.number().min(0).max(3), which silently rejected any saved shift value greater than 3 (the slider goes up to 7, the number input up to 10). It also had no path for older Z-Image images that don't include z_image_shift in their metadata, so "Use All" left the current shift untouched instead of resetting to auto.

The handler now:

• accepts 0..10 to match ParamZImageShift's numberInputMax,
• resolves to null when the key is null or absent and the active base is z-image (checked via selectBase, which is reliable because MainModel is sorted to recall first),
• rejects on absent key for non-Z-Image recalls so unrelated metadata doesn't clobber the user's current shift.

Related Issues / Discussions

Closes #9135

QA Instructions

1. Generate a Z-Image with shift > 3 (e.g. 5) → open the image in the gallery → click Use All. Expected: shift slider recalls to 5. Before this fix it stayed on whatever value was set in the UI.
2. Generate a Z-Image with shift left at auto → Use All. Expected: shift recalls to auto.
3. Open an older Z-Image (generated before z_image_shift was added to metadata) → Use All. Expected: shift recalls to auto.
4. While on Z-Image with a custom shift, Use All on a non-Z-Image (SDXL / FLUX / Qwen). Expected: Z-Image shift is not reset (handler rejects when base isn't z-image).
5. Hover the metadata viewer on an image with shift = auto. Expected: value renders as Auto (was previously empty/undefined).

Merge Plan

Standard merge — no schema or migration impact, redux slice shape is unchanged (zImageShift: number | null already supports null).

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)