feat(model-manager): base-model multi-select filter on CivitAI browse

Add a checkbox dropdown to filter the CivitAI catalog by one or more base
models (SD 1.5, SDXL, Pony, Illustrious, Flux, etc.), mapped to the API's
`baseModels` array param. Backend search accepts comma-separated
base_models; button shows the selected count with a Clear action.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -239,7 +239,8 @@ SparkyUI includes a **StabilityMatrix-style Model Manager** - a lightweight Fast
   click for a full-size lightbox view, and **permanently delete** photos one at a time or
   all at once (with confirm).
 - **Browse CivitAI** - search the CivitAI catalog in a thumbnail grid (filter by type,
-  sort, period, NSFW toggle) and **click a model to download it** - no URL pasting needed.
+  base model (multi-select), sort, period, NSFW toggle) and **click a model to download
+  it** - no URL pasting needed.
   Multi-version models get a version picker on the card. **Early Access** versions are
   flagged (they require purchased access on CivitAI and otherwise fail with 401).
 - **Installed Models** - browse what's on disk, grouped by type, with size and delete actions