feat(model-manager): "Free GPU memory" button to unload ComfyUI models

ComfyUI caches the last model when RAM is plentiful (unified memory), so
memory doesn't drop after switching models even though models are being
swapped, not accumulated. Add a sidebar "Free GPU memory" button that
proxies ComfyUI's POST /free (unload_models + free_memory) via a new
/api/comfyui/free endpoint (COMFYUI_URL env). Verified it releases ~7GB.
README documents this plus the --disable-smart-memory auto-unload option.

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

model-manager/app/static/index.html

@@ -33,7 +33,12 @@
           <span class="nav-ico">⚙</span> Settings
         </button>
       </nav>
-      <div class="sidebar-foot" id="modelsDir"></div>
+      <div class="sidebar-foot">
+        <button class="btn" id="freeMemBtn" title="Unload all models from ComfyUI and free GPU/RAM">
+          ⏏ Free GPU memory
+        </button>
+        <span class="form-msg" id="freeMemMsg"></span>
+      </div>
     </aside>
 
     <main class="content">