feat(model-manager): add "Delete all" gallery button + clearer 401 errors

- Gallery: DELETE /api/gallery/all removes every image under output/;
  "Delete all" button with in-app confirm and a deleted/failed count.
- Downloads: surface a clear, actionable message when CivitAI/HuggingFace
  returns 401/403 (model requires login/early-access, or the key/token
  lacks access) instead of a bare error, both at resolve time and during
  the download stream.

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

README.md

@@ -236,7 +236,8 @@ SparkyUI includes a **StabilityMatrix-style Model Manager** - a lightweight Fast
 
 **Features:**
 - **Gallery** - browse generated photos from ComfyUI's `output/` in a large desktop grid,
-  click for a full-size lightbox view, and **permanently delete** photos (with confirm).
+  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.
   Multi-version models get a version picker on the card.