Feature (UI): add model path update for external models

[Pfannkuchensack]

Summary

Adds ability to update the file path for external models in the Model Manager.

What:

• New "Update Path" button in the model details view
• Modal dialog to enter a new path for the model
• Button is only shown for external models (models with absolute paths like D:/Models/model.safetensors)
• Invoke-controlled models (relative paths like uuid/model.safetensors) are excluded

Why:
Users who manage their model files externally may move them to different locations on disk. This feature allows updating the path in InvokeAI's database without having to remove and re-add the model.

How:

• Added isExternalModel() helper that checks if a path is absolute (Unix /... or Windows X:/...)
• ModelUpdatePathButton component with modal for path input
• Uses existing useUpdateModelMutation API

Related Issues / Discussions

QA Instructions

1. Add an external model (with absolute path) via "Add Model" → "Path"
2. Open the model in Model Manager
3. Verify "Update Path" button is visible
4. Click button, enter new path, save
5. Verify path is updated

Negative test:

1. Install a model via HuggingFace (Invoke-controlled)
2. Open the model in Model Manager
3. Verify "Update Path" button is NOT visible

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)

[Pfannkuchensack]

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