Fix shared view localStorage interference and update OG preview

Block persistentAtom from writing shared token responses to localStorage
by overriding setItem for the session. Update OG preview image to show
realistic heatmap with correct/incorrect answer dots.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jeremymanning

public/og-preview.png

@@ -946,20 +946,24 @@ function injectSharedResponses(responses) {
   globalEstimator.reset();
   globalEstimator.init(GLOBAL_GRID_SIZE, GLOBAL_REGION);
 
-  // Back up the user's own responses before overwriting with shared data
+  // Save the user's own responses before we touch the store
   const ownResponses = localStorage.getItem('mapper:responses');
-  if (ownResponses) {
-    localStorage.setItem('mapper:responses:backup', ownResponses);
-  }
 
-  // Set responses on the store (overrides any local responses)
+  // Prevent persistentAtom from writing shared responses to localStorage.
+  // Block all writes to 'mapper:responses' for the entire shared view session,
+  // since subscriptions/computed atoms may trigger re-persists at any time.
+  const origSetItem = localStorage.setItem.bind(localStorage);
+  localStorage.setItem = function(key, value) {
+    if (key === 'mapper:responses') return; // block shared data from persisting
+    origSetItem(key, value);
+  };
+
+  // Set responses on the store (updates in-memory only, localStorage write is blocked)
   $responses.set(responses || []);
 
-  // Restore the backup so navigating away doesn't lose the user's own progress
+  // Ensure user's own data stays intact in localStorage
   if (ownResponses) {
-    localStorage.setItem('mapper:responses', ownResponses);
-  } else {
-    localStorage.removeItem('mapper:responses');
+    origSetItem('mapper:responses', ownResponses);
   }
 
   if (!responses || responses.length === 0) {