feat(image-ref): Bild-Referenz Widget mit Session-Storage

Opt-in Widget fuer Bild-Referenzen (max. 3 gleichzeitig).
Canvas API konvertiert zu WebP, sessionStorage fuer Bilddaten.
Positionen und Labels bleiben persistent, Bilder nur pro Session.

newtab.html

@@ -73,6 +73,9 @@
     <button class="widget-toolbar-btn" data-action="timer" title="Timer">
       <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><circle cx="12" cy="13" r="8"/><path d="M12 9v4l2 2"/><path d="M5 3l2 2"/><path d="M19 3l-2 2"/><line x1="12" y1="1" x2="12" y2="3"/></svg>
     </button>
+    <button class="widget-toolbar-btn hidden" data-action="image-ref" title="Bild-Referenz">
+      <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><rect x="3" y="3" width="18" height="18" rx="2"/><circle cx="8.5" cy="8.5" r="1.5"/><path d="M21 15l-5-5L5 21"/></svg>
+    </button>
     <button class="widget-toolbar-btn" data-action="notebook" title="Alle Notes">
       <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M4 19.5A2.5 2.5 0 016.5 17H20"/><path d="M6.5 2H20v20H6.5A2.5 2.5 0 014 19.5v-15A2.5 2.5 0 016.5 2z"/></svg>
     </button>
@@ -197,6 +200,16 @@
               <option value="left">Links</option>
             </select>
           </div>
+          <div class="setting-row">
+            <div class="setting-info">
+              <span class="setting-label">Bild-Referenz Widgets</span>
+              <span class="setting-desc">Bilder als Referenz anzeigen (nur aktuelle Session)</span>
+            </div>
+            <label class="toggle">
+              <input type="checkbox" id="settingImageRef">
+              <span class="slider"></span>
+            </label>
+          </div>
         </div>
       </section>
 
@@ -483,6 +496,7 @@
   <script src="src/js/notes.js"></script>
   <script src="src/js/calculator.js"></script>
   <script src="src/js/timer.js"></script>
+  <script src="src/js/image-ref.js"></script>
   <script src="src/js/data.js"></script>
   <!-- Onboarding -->
   <script src="src/js/onboarding.js"></script>