feat: live preview toggle, FPS indicator, live counting by object

- Toggle ON/OFF switch to disable preview (faster processing)
- FPS badge showing rolling average processing speed
- Live Loading/Unloading/Net counts updated in real-time
- Pipeline callback pushes stats every 10 frames
- Frontend polls and updates display every 2s
- 62/62 tests pass

static/style.css

@@ -660,6 +660,99 @@ main {
   display: block;
 }
 
+.live-preview-header {
+  display: flex;
+  align-items: center;
+  gap: var(--space-3);
+  margin-bottom: var(--space-3);
+}
+
+.live-preview-header .section-title {
+  margin-bottom: 0;
+}
+
+.live-fps {
+  font-size: var(--text-xs);
+  font-weight: var(--font-semibold);
+  color: var(--color-primary);
+  background: var(--color-primary-50, #f0fdfa);
+  padding: 2px 8px;
+  border-radius: var(--radius-sm);
+  white-space: nowrap;
+}
+
+.toggle-label {
+  display: flex;
+  align-items: center;
+  gap: var(--space-2);
+  cursor: pointer;
+  font-size: var(--text-sm);
+  user-select: none;
+}
+
+.toggle-label input[type="checkbox"] {
+  display: none;
+}
+
+.toggle-switch {
+  width: 36px;
+  height: 20px;
+  background: var(--color-neutral-300);
+  border-radius: 10px;
+  position: relative;
+  transition: background var(--transition-fast);
+}
+
+.toggle-switch::after {
+  content: '';
+  position: absolute;
+  width: 16px;
+  height: 16px;
+  background: white;
+  border-radius: 50%;
+  top: 2px;
+  left: 2px;
+  transition: transform var(--transition-fast);
+}
+
+.toggle-label input:checked + .toggle-switch {
+  background: var(--color-primary);
+}
+
+.toggle-label input:checked + .toggle-switch::after {
+  transform: translateX(16px);
+}
+
+.toggle-text {
+  font-weight: var(--font-medium);
+  color: var(--color-neutral-600);
+}
+
+.live-counts {
+  display: flex;
+  gap: var(--space-6);
+  padding: var(--space-3) 0;
+  border-top: 1px solid var(--color-neutral-100);
+  margin-top: var(--space-3);
+}
+
+.live-count-item {
+  text-align: center;
+  flex: 1;
+}
+
+.live-count-label {
+  font-size: var(--text-xs);
+  color: var(--color-neutral-500);
+  display: block;
+  margin-bottom: var(--space-1);
+}
+
+.live-count-value {
+  font-size: var(--text-2xl);
+  font-weight: var(--font-bold);
+}
+
 .preview-placeholder {
   position: absolute;
   inset: 0;