[Feat] True Mask Opacity Slider

### What problem are you experiencing?

To draw a mask with partial opacity, you must use the brush's opacity setting and manually draw the opacity using the brush and eraser. The overall opacity can't be fine-tuned.

This is especially useful in image-to-image workflows, where some features of the rough input image should be partially preserved. A solid mask preserves all the roughness, but no mask destroys the feature. A partial mask is the solution.

### When does this problem occur?

When using the mask editor.

### How often do you encounter this problem?

Multiple times per day

### How much does this problem affect your workflow?

Significantly slows down my work

### Current workarounds

Painstakingly trial-and-error 5-7 generations to get the correct opacity.

### Ideas for solutions (Optional)

A slider that affects the mask opacity by multiplying it by a number from 0-1.

### Additional context

The current "Mask Opacity" slider does not affect the final mask, it only acts on the preview. True opacity can only be achieved with the brush as of right now.

Start in the mask editor and trace how the existing “Mask Opacity” slider updates the preview versus the final mask. Add the requested 0–1 multiplier to the final mask behavior, then verify that partial-opacity masks produce the intended image-to-image blend.

Written by the indexing model from the issue text.