fix(ui): make Z-Image model selects mutually exclusive

[Pfannkuchensack]

Summary

Makes the Z-Image model selection inputs mutually exclusive:

• When Qwen3 Source is selected, VAE and Qwen3 Encoder selects are disabled
• When VAE or Qwen3 Encoder is selected, Qwen3 Source select is disabled

This prevents users from selecting invalid model combinations.

Additionally, when switching to a Z-Image model, valid defaults are automatically set if no configuration exists:

• Prefers Qwen3 Source (Diffusers model) if available
• Falls back to Qwen3 Encoder + FLUX VAE combination

This ensures the generate button is enabled immediately after selecting a Z-Image model.

Related Issues / Discussions

QA Instructions

1. Select a Z-Image model
2. Verify that a valid Qwen3 configuration is automatically selected
3. Open Advanced Settings
4. Test the mutual exclusion:
   • Select a VAE or Qwen3 Encoder → Qwen3 Source should be disabled
   • Clear the selection, then select a Qwen3 Source → VAE and Qwen3 Encoder should be disabled
5. Reset the UI and select Z-Image-Turbo → Generate button should be enabled immediately

Merge Plan

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)

[lstein]

It works as expected.

Change requested: When the user interface is reset and you select Z-Image-Turbo as the main model, the render button remains disabled until the user goes to Advanced and selects one of the Qwen3 Encoder options. This also happens when remixing a previous image. Can we default to a valid Qwen3 source? I was going to request this in a separate issue, but this seems like an appropriate PR for a fix.

The only other gotcha that I encountered was that when I switched from main to this PR and reloaded the browser page, I found the UI in a state in which all the Advanced menus were disabled and I was unable to reenable them by refreshing the page, recalling parameters, or switching main models. The problem was addressed by resetting the Web UI. I have tried to reproduce the problem, but haven't been able to do so, so probably ok to ignore.

[Pfannkuchensack]

Added the automatically select of Qwen3 Source (Diffusers model) if available and Falls back to Qwen3 Encoder + FLUX VAE combination

[Pfannkuchensack]

Fixed the #8721 issue too

[lstein]

Tested working.