remove: source passage section and Open Q/MCQ mode toggle from UI

ui/index.html

@@ -389,11 +389,6 @@
 
       <!-- Controls row -->
       <div class="controls-row">
-        <!-- Mode toggle -->
-        <div class="toggle-group">
-          <button class="pill active" id="mode-open-btn">Open Q</button>
-          <button class="pill" id="mode-mcq-btn">MCQ</button>
-        </div>
         <!-- Score ring -->
         <div class="score-wrap" style="margin-left:auto" title="Score">
           <svg class="score-ring" viewBox="0 0 56 56">
@@ -474,10 +469,6 @@
       </div>
       <div class="feedback-body" id="feedback-body"></div>
       <div class="mcq-explanations hidden" id="mcq-explanations"></div>
-      <details class="source-details" id="source-details">
-        <summary>View source passage</summary>
-        <p class="source-text" id="source-text"></p>
-      </details>
     </div>
   </div>