Preserve local paths for desktop saves (#5543)

# Summary

- Adds desktop file tracking: local paths are preserved and save buttons
now work as expcted (doing Save/Save As as appropriate)
- Adds logic to track whether files are 'dirty' (they've been modified
by some tool, and not saved to disk yet).
- Improves file state UX (dirty vs saved) and close warnings
- Web behaviour should be unaffected by these changes

## Indicators
Files now have indicators in desktop mode to tell you their state.

### File up-to-date with disk

<img width="318" height="393" alt="image"
src="https://github.com/user-attachments/assets/06325f9a-afd7-4c2f-8a5b-6d11e3093115"
/>

### File modified by a tool but not saved to disk yet

<img width="357" height="385" alt="image"
src="https://github.com/user-attachments/assets/1a7716d9-c6f7-4d13-be0d-c1de6493954b"
/>

### File not tracked on disk

<img width="312" height="379" alt="image"
src="https://github.com/user-attachments/assets/9cffe300-bd9a-4e19-97c7-9b98bebefacc"
/>

# Limitations
- It's a bit weird that we still have files stored in indexeddb in the
app, which are still loadable. We might want to change this behaviour in
the future
- Viewer's Save doesn't persist to disk. I've left that out here because
it'd need a lot of testing to make sure the logic's right with making
sure you can leave the Viewer with applying the changes to the PDF
_without_ saving to disk
- There's no current way to do Save As on a file that has already been
persisted to disk - it's only ever Save. Similarly, there's no way to
duplicate a file.

---------

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

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

@@ -17,6 +17,12 @@ confirmClose = "Confirm Close"
 confirmCloseCancel = "Cancel"
 confirmCloseConfirm = "Close File"
 confirmCloseMessage = "Are you sure you want to close this file?"
+confirmCloseDiscard = "Discard changes and close"
+confirmCloseSave = "Save and close"
+confirmCloseUnsaved = "This file has unsaved changes."
+confirmCloseUnsavedList = "You have {{count}} file{{plural}} with unsaved changes.\n\n{{fileList}}"
+confirmCloseSaveFailedTitle = "Save Failed"
+confirmCloseSaveFailed = "Saved with errors. {{count}} file{{plural}} could not be saved."
 confirmPasswordErrorMessage = "New Password and Confirm New Password must match."
 custom = "Custom..."
 customPosition = "Custom Position"
@@ -39,6 +45,8 @@ edit = "Edit"
 editYourNewFiles = "Edit your new file(s)"
 exportAndContinue = "Export & Continue"
 false = "False"
+fileSavedToDisk = "File saved to disk"
+fileNotSavedToDisk = "Not saved to disk"
 fileSelected = "Selected: {{filename}}"
 filesSelected = "{{count}} files selected"
 font = "Font"