Feature/v2/compare tool (#4751)

# Description of Changes

- Addition of the compare tool
- 
---

## 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: James Brunton <[email protected]>

frontend/src/core/components/viewer/useViewerRightRailButtons.tsx

@@ -62,8 +62,8 @@ export function useViewerRightRailButtons() {
         render: ({ disabled }) => (
           <Tooltip content={panLabel} position="left" offset={12} arrow portalTarget={document.body}>
             <ActionIcon
-              variant={isPanning ? 'filled' : 'subtle'}
-              color={isPanning ? 'blue' : undefined}
+              variant={isPanning ? 'default' : 'subtle'}
+              color={undefined}
               radius="md"
               className="right-rail-icon"
               onClick={() => {
@@ -71,6 +71,7 @@ export function useViewerRightRailButtons() {
                 setIsPanning(prev => !prev);
               }}
               disabled={disabled}
+              style={isPanning ? { backgroundColor: 'var(--right-rail-pan-active-bg)' } : undefined}
             >
               <LocalIcon icon="pan-tool-rounded" width="1.5rem" height="1.5rem" />
             </ActionIcon>