feat(model-picker): group models by [category] name prefix

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Summary

Models whose display name starts with [category]… are now grouped by that category in the model picker. For the main model picker this adds an inline subgroup header within each base group (FLUX, SDXL, …); for the LoRA picker (flat list) categories become the top-level groups with toggle buttons. Uncategorized models appear after the categorized ones without a header.

The [category] prefix is hidden in the picker list and trigger button but kept intact in the underlying model name, so the Model Manager still shows the full name.

Example: renaming a model to [flux]flux.1 dev puts it under a "flux" subgroup inside the FLUX base group; the option itself is displayed as flux.1 dev.

Related Issues / Discussions

Addresses #4592 (Model Manager — Grouping checkpoint models by category).

QA Instructions

1. Open the Model Manager and rename a couple of installed models so their names start with a bracket prefix, e.g. [realistic]My Model A, [realistic]My Model B, [anime]My Model C. Leave at least one model in the same base without a prefix.
2. Open the main model picker (Generation Settings).
   • Within the model's base group (e.g. SDXL), confirm a small realistic subheader appears above the two realistic models and an anime subheader above the anime one.
   • Confirm the uncategorized model appears after the categorized ones, without a header.
   • Confirm option labels and the picker trigger button show the cleaned name (no […] prefix).
   • Type realistic into the search — only the realistic models should remain.
3. Pick a categorized model and open the Model Manager → confirm the full original name (including [realistic]…) is still shown there.
4. Add a few LoRAs whose names start with a bracket prefix, then open the LoRA picker.
   • Confirm top-level groups appear per category, plus an Uncategorized group at the end if applicable.
   • Confirm group toggle badges work as filters.
   • With no categorized LoRAs available, the picker should fall back to its previous flat-list behavior.

Merge Plan

No special steps. No DB / redux migrations. Touches frontend only.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable) — added modelPickerCategory.test.ts covering the name-prefix parser
• ❗Changes to a redux slice have a corresponding migration — n/a
• Documentation added / updated (if applicable) — n/a
• Updated What's New copy (if doing a release after this PR)

[Pfannkuchensack]

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