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

templates/status.html

@@ -33,11 +33,33 @@
   </div>
 
   <div class="live-preview-section card" id="live-preview-section" style="display: block">
-    <h3 class="section-title">Live Preview</h3>
+    <div class="live-preview-header">
+      <h3 class="section-title">Live Preview</h3>
+      <label class="toggle-label" for="preview-toggle">
+        <input type="checkbox" id="preview-toggle" checked>
+        <span class="toggle-switch"></span>
+        <span class="toggle-text" id="toggle-text">ON</span>
+      </label>
+      <span class="live-fps" id="live-fps">-- FPS</span>
+    </div>
     <div class="live-preview-wrap">
       <img id="live-preview-img" class="live-preview-img" src="/api/preview/{{ job.job_id }}" alt="Annotated frame from video processing" />
       <div class="preview-placeholder" id="preview-placeholder">Waiting for first frame...</div>
     </div>
+    <div class="live-counts" id="live-counts" style="display:none">
+      <div class="live-count-item">
+        <span class="live-count-label">Loading</span>
+        <span class="live-count-value stat-loading" id="live-loading">0</span>
+      </div>
+      <div class="live-count-item">
+        <span class="live-count-label">Unloading</span>
+        <span class="live-count-value stat-unloading" id="live-unloading">0</span>
+      </div>
+      <div class="live-count-item">
+        <span class="live-count-label">Net</span>
+        <span class="live-count-value stat-net" id="live-net">0</span>
+      </div>
+    </div>
   </div>
 
   <div class="video-thumb-section card" id="video-thumb-section">