Fix: clicking on history item now shows diff for that revision

- Modified ui.js displayHistory() to pass previous revision to click handler
- Added showHistoryItemDiff() method in app.js to generate and display diff
- Diff compares selected revision with previous revision (Option A)
- First revision shows message instead of diff (no previous to compare)

ui/ui.js

@@ -254,14 +254,19 @@ class UIManager {
         this.showView(this.historyView);
 
         // Add click handlers to history items
-        this.historyContent.querySelectorAll('.history-item').forEach(item => {
+        this.historyContent.querySelectorAll('.history-item').forEach((item, index) => {
             item.addEventListener('click', () => {
                 // Remove active class from all items
                 this.historyContent.querySelectorAll('.history-item').forEach(i => i.classList.remove('active'));
                 // Add active class to clicked item
                 item.classList.add('active');
                 const revision = item.dataset.revision;
-                window.app.loadFileAtRevision(this.currentFile, revision);
+                
+                // Get the previous revision if it exists
+                const previousRevision = index < history.length - 1 ? history[index + 1].revision : null;
+                
+                // Show diff between current and previous revision
+                window.app.showHistoryItemDiff(this.currentFile, revision, previousRevision);
             });
         });
     }