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/src/desktop/components/AppProviders.tsx

@@ -2,6 +2,7 @@ import { ReactNode, useEffect, useState } from "react";
 import { AppProviders as ProprietaryAppProviders } from "@proprietary/components/AppProviders";
 import { DesktopConfigSync } from '@app/components/DesktopConfigSync';
 import { DesktopBannerInitializer } from '@app/components/DesktopBannerInitializer';
+import { SaveShortcutListener } from '@app/components/SaveShortcutListener';
 import { SetupWizard } from '@app/components/SetupWizard';
 import { useFirstLaunchCheck } from '@app/hooks/useFirstLaunchCheck';
 import { useBackendInitializer } from '@app/hooks/useBackendInitializer';
@@ -149,6 +150,7 @@ export function AppProviders({ children }: { children: ReactNode }) {
     >
       <DesktopConfigSync />
       <DesktopBannerInitializer />
+      <SaveShortcutListener />
       {children}
     </ProprietaryAppProviders>
   );