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/core/services/openFilesFromDisk.ts

@@ -0,0 +1,29 @@
+import { openFileDialog } from '@app/services/fileDialogService';
+import { pendingFilePathMappings } from '@app/contexts/FileManagerContext';
+import { getDocumentFileDialogFilter } from '@app/utils/fileDialogUtils';
+
+interface OpenFilesFromDiskOptions {
+  multiple?: boolean;
+  filters?: Array<{
+    name: string;
+    extensions: string[];
+  }>;
+  onFallbackOpen?: () => void;
+}
+
+export async function openFilesFromDisk(options: OpenFilesFromDiskOptions = {}): Promise<File[]> {
+  const filesWithPaths = await openFileDialog({
+    multiple: options.multiple ?? true,
+    filters: options.filters ?? getDocumentFileDialogFilter()
+  });
+
+  if (filesWithPaths.length > 0) {
+    for (const { quickKey, path } of filesWithPaths) {
+      pendingFilePathMappings.set(quickKey, path);
+    }
+    return filesWithPaths.map((entry) => entry.file);
+  }
+
+  options.onFallbackOpen?.();
+  return [];
+}