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"
/>


---

### AI Usage

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

Closes #754

src/activities/reader/KOReaderSyncActivity.cpp

@@ -317,7 +317,6 @@ void KOReaderSyncActivity::render() {
              localProgress.percentage * 100);
     renderer.drawText(UI_10_FONT_ID, 20, 320, localPageStr);
 
-    // Options
     const int optionY = 350;
     const int optionHeight = 30;
 
@@ -333,13 +332,8 @@ void KOReaderSyncActivity::render() {
     }
     renderer.drawText(UI_10_FONT_ID, 20, optionY + optionHeight, "Upload local progress", selectedOption != 1);
 
-    // Cancel option
-    if (selectedOption == 2) {
-      renderer.fillRect(0, optionY + optionHeight * 2 - 2, pageWidth - 1, optionHeight);
-    }
-    renderer.drawText(UI_10_FONT_ID, 20, optionY + optionHeight * 2, "Cancel", selectedOption != 2);
-
-    const auto labels = mappedInput.mapLabels("", "Select", "", "");
+    // Bottom button hints: show Back and Select
+    const auto labels = mappedInput.mapLabels("Back", "Select", "", "");
     GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
     renderer.displayBuffer();
     return;
@@ -349,7 +343,7 @@ void KOReaderSyncActivity::render() {
     renderer.drawCenteredText(UI_10_FONT_ID, 280, "No remote progress found", true, EpdFontFamily::BOLD);
     renderer.drawCenteredText(UI_10_FONT_ID, 320, "Upload current position?");
 
-    const auto labels = mappedInput.mapLabels("Cancel", "Upload", "", "");
+    const auto labels = mappedInput.mapLabels("Back", "Upload", "", "");
     GUI.drawButtonHints(renderer, labels.btn1, labels.btn2, labels.btn3, labels.btn4);
     renderer.displayBuffer();
     return;
@@ -392,11 +386,11 @@ void KOReaderSyncActivity::loop() {
     // Navigate options
     if (mappedInput.wasPressed(MappedInputManager::Button::Up) ||
         mappedInput.wasPressed(MappedInputManager::Button::Left)) {
-      selectedOption = (selectedOption + 2) % 3;  // Wrap around
+      selectedOption = (selectedOption + 1) % 2;  // Wrap around among 2 options
       updateRequired = true;
     } else if (mappedInput.wasPressed(MappedInputManager::Button::Down) ||
                mappedInput.wasPressed(MappedInputManager::Button::Right)) {
-      selectedOption = (selectedOption + 1) % 3;
+      selectedOption = (selectedOption + 1) % 2;  // Wrap around among 2 options
       updateRequired = true;
     }
 
@@ -407,9 +401,6 @@ void KOReaderSyncActivity::loop() {
       } else if (selectedOption == 1) {
         // Upload local progress
         performUpload();
-      } else {
-        // Cancel
-        onCancel();
       }
     }
 

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;