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>

model-manager/app/static/index.html

@@ -104,6 +104,15 @@
             <option>Week</option>
             <option>Day</option>
           </select>
+          <div class="multi-dd" id="bBaseDD">
+            <button type="button" class="btn multi-btn" id="bBaseBtn">Base model ▾</button>
+            <div class="multi-panel" id="bBasePanel" hidden>
+              <div class="multi-panel-actions">
+                <button type="button" class="link-btn" id="bBaseClear">Clear</button>
+              </div>
+              <!-- checkboxes injected by app.js -->
+            </div>
+          </div>
           <label class="check"><input type="checkbox" id="bNsfw" /> NSFW</label>
           <button class="btn primary" id="bGo">Search</button>
         </div>