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>

model-manager/app/static/index.html

@@ -58,6 +58,7 @@
           <div class="head-actions">
             <span class="form-msg" id="galleryToast"></span>
             <button class="btn" id="refreshGallery">Refresh</button>
+            <button class="btn danger" id="deleteAllGallery">Delete all</button>
           </div>
         </header>
         <div id="galleryGrid" class="gallery-grid">