Handle non-pdf gracefully in viewer (#5004)

# Description of Changes

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- Why the change was made
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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)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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.

frontend/public/locales/en-GB/translation.json

@@ -3890,14 +3890,17 @@
     "actualSize": "Actual Size"
   },
   "viewer": {
+    "cannotPreviewFile": "Cannot Preview File",
+    "dualPageView": "Dual Page View",
     "firstPage": "First Page",
     "lastPage": "Last Page",
-    "previousPage": "Previous Page",
     "nextPage": "Next Page",
-    "zoomIn": "Zoom In",
-    "zoomOut": "Zoom Out",
+    "onlyPdfSupported": "The viewer only supports PDF files. This file appears to be a different format.",
+    "previousPage": "Previous Page",
     "singlePageView": "Single Page View",
-    "dualPageView": "Dual Page View"
+    "unknownFile": "Unknown file",
+    "zoomIn": "Zoom In",
+    "zoomOut": "Zoom Out"
   },
   "rightRail": {
     "closeSelected": "Close Selected Files",