Feature/v2/viewer tabs (#4646)

# Description of Changes

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Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
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---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: James Brunton <[email protected]>
Co-authored-by: Claude <[email protected]>

frontend/src/contexts/ViewerContext.tsx

@@ -132,6 +132,10 @@ interface ViewerContextType {
   setAnnotationMode: (enabled: boolean) => void;
   toggleAnnotationMode: () => void;
 
+  // Active file index for multi-file viewing
+  activeFileIndex: number;
+  setActiveFileIndex: (index: number) => void;
+
   // State getters - read current state from bridges
   getScrollState: () => ScrollState;
   getZoomState: () => ZoomState;
@@ -219,6 +223,7 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
   const [isThumbnailSidebarVisible, setIsThumbnailSidebarVisible] = useState(false);
   const [isAnnotationsVisible, setIsAnnotationsVisible] = useState(true);
   const [isAnnotationMode, setIsAnnotationModeState] = useState(false);
+  const [activeFileIndex, setActiveFileIndex] = useState(0);
 
   // Get current navigation state to check if we're in sign mode
   useNavigation();
@@ -577,6 +582,10 @@ export const ViewerProvider: React.FC<ViewerProviderProps> = ({ children }) => {
     setAnnotationMode,
     toggleAnnotationMode,
 
+    // Active file index
+    activeFileIndex,
+    setActiveFileIndex,
+
     // State getters
     getScrollState,
     getZoomState,