fix(ui): Add Back label in KOReader Sync screen (#770)

## Summary

- Remove duplicate Cancel option 
- Add Back label

<img width="435" height="613" alt="image"
src="https://github.com/user-attachments/assets/a3af4133-46fa-46e6-8360-a15dd7c4fe2a"
/>


## Result

<img width="575" height="431" alt="image"
src="https://github.com/user-attachments/assets/6ccdac89-43df-45bf-bcfa-3a7cc4bd88e4"
/>


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### AI Usage

Did you use AI tools to help write this code? _**< PARTIALLY >**_

Closes #754

src/activities/reader/KOReaderSyncActivity.h

@@ -18,7 +18,7 @@
  * 1. Connect to WiFi (if not connected)
  * 2. Calculate document hash
  * 3. Fetch remote progress
- * 4. Show comparison and options (Apply/Upload/Cancel)
+ * 4. Show comparison and options (Apply/Upload)
  * 5. Apply or upload progress
  */
 class KOReaderSyncActivity final : public ActivityWithSubactivity {
@@ -82,7 +82,7 @@ class KOReaderSyncActivity final : public ActivityWithSubactivity {
   // Local progress as KOReader format (for display)
   KOReaderPosition localProgress;
 
-  // Selection in result screen (0=Apply, 1=Upload, 2=Cancel)
+  // Selection in result screen (0=Apply, 1=Upload)
   int selectedOption = 0;
 
   OnCancelCallback onCancel;