fix: always show Scan button in Serve tab; invalidate cache after download

- Added persistent Scan button to the Serve tab toolbar (was only shown
  in empty state, making it impossible to refresh when localStorage had
  cached results)
- Hooked into background poll to clear model scan cache (key:
  cookbook_cached_models_scan_v1) when downloads transition to done,
  so the Serve tab doesn't show stale data after a download completes

Event handler for #hwfit-cache-scan already existed in the codebase but
the button element was never rendered in the HTML template.

static/js/cookbook.js

@@ -2802,6 +2802,7 @@ function _renderRecipes() {
   html += '<div class="memory-category-filters">';
   html += '<input type="text" class="memory-search-input" id="serve-search" placeholder="Search cached models\u2026" style="flex:1;min-width:120px;" />';
   html += '<button class="memory-toolbar-btn" id="hwfit-cache-select">Select</button>';
+  html += '<button class="memory-toolbar-btn" id="hwfit-cache-scan" title="Rescan model cache" style="margin-left:4px;">Scan</button>';
   html += '</div>';
   html += '<div class="doclib-lang-chips" id="serve-tags"></div>';
   html += '</div>';

static/js/cookbookRunning.js

@@ -3948,6 +3948,12 @@ async function _pollBackgroundStatus() {
           _showDiagnosis(el, task._backendDiagnosis, task.output || '');
         }
         completedDeps.forEach(t => _refreshDepsAfterInstall(t));
+        // Ulysses: when a download finishes, invalidate the model scan cache
+        // so the Serve tab doesn't show stale results.
+        const justDownloaded = localTasks.filter(t => t.type === 'download' && t.status === 'done');
+        if (justDownloaded.length) {
+          try { localStorage.removeItem('cookbook_cached_models_scan_v1'); } catch {}
+        }
       }
     } catch (_) { /* non-fatal: background status should never break polling */ }