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/downloadService.ts

@@ -0,0 +1,40 @@
+export interface DownloadRequest {
+  data: Blob | File;
+  filename: string;
+  localPath?: string;
+}
+
+export interface DownloadResult {
+  savedPath?: string;
+  cancelled?: boolean;
+}
+
+export async function downloadFile(request: DownloadRequest): Promise<DownloadResult> {
+  const url = URL.createObjectURL(request.data);
+
+  const link = document.createElement("a");
+  link.href = url;
+  link.download = request.filename;
+  document.body.appendChild(link);
+  link.click();
+  document.body.removeChild(link);
+
+  URL.revokeObjectURL(url);
+
+  return { savedPath: request.localPath };
+}
+
+export async function downloadFromUrl(
+  url: string,
+  filename: string,
+  localPath?: string
+): Promise<DownloadResult> {
+  const link = document.createElement("a");
+  link.href = url;
+  link.download = filename;
+  document.body.appendChild(link);
+  link.click();
+  document.body.removeChild(link);
+
+  return { savedPath: localPath };
+}

frontend/src/core/services/fileDialogService.ts

@@ -0,0 +1,28 @@
+// Core stub - no-op implementation for web builds
+// Desktop overrides this with actual Tauri implementation
+
+export interface FileWithPath {
+  file: File;
+  path: string;
+  quickKey: string;
+}
+
+export interface FileDialogOptions {
+  multiple?: boolean;
+  filters?: Array<{
+    name: string;
+    extensions: string[];
+  }>;
+}
+
+/**
+ * Open native file dialog and read selected files
+ * Core stub - returns empty array (no native dialog in web)
+ * Desktop builds override this with actual Tauri implementation
+ */
+export async function openFileDialog(
+  _options?: FileDialogOptions
+): Promise<FileWithPath[]> {
+  // Web build: no native file dialog support
+  return [];
+}

frontend/src/core/services/localFileSaveService.ts

@@ -0,0 +1,48 @@
+// Core stub - no-op implementation for web builds
+// Desktop overrides this with actual Tauri implementation
+
+export interface SaveResult {
+  success: boolean;
+  error?: string;
+}
+
+export interface MultiFileSaveResult {
+  success: boolean;
+  savedCount: number;
+  cancelledByUser?: boolean;
+  error?: string;
+}
+
+/**
+ * Save file data to a local filesystem path
+ * Core stub - always returns failure
+ * Desktop builds override this with actual implementation
+ */
+export async function saveToLocalPath(
+  _data: Blob | File,
+  _filePath: string
+): Promise<SaveResult> {
+  return { success: false, error: "Local file save not available in web mode" };
+}
+
+/**
+ * Show native save dialog
+ * Core stub - always returns null
+ */
+export async function showSaveDialog(
+  _defaultFilename: string,
+  _defaultDirectory?: string
+): Promise<string | null> {
+  return null;
+}
+
+/**
+ * Prompt user to select folder and save multiple files
+ * Core stub - always returns failure
+ */
+export async function saveMultipleFilesWithPrompt(
+  _files: (Blob | File)[],
+  _defaultDirectory?: string
+): Promise<MultiFileSaveResult> {
+  return { success: false, savedCount: 0, error: "Multi-file save not available in web mode" };
+}

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 [];
+}

frontend/src/core/services/operationResultsSaveService.ts

@@ -0,0 +1,30 @@
+import type { FileId, StirlingFileStub } from '@app/types/fileContext';
+import { downloadFromUrl, DownloadResult } from '@app/services/downloadService';
+
+export interface OperationSaveContext {
+  downloadUrl: string | null;
+  downloadFilename: string;
+  downloadLocalPath?: string | null;
+  outputFileIds?: string[] | null;
+  getFile: (fileId: FileId) => File | undefined;
+  getStub: (fileId: FileId) => StirlingFileStub | undefined;
+  markSaved: (fileId: FileId, savedPath?: string) => void;
+}
+
+export async function saveOperationResults(context: OperationSaveContext): Promise<DownloadResult | null> {
+  if (!context.downloadUrl) return null;
+
+  const result = await downloadFromUrl(
+    context.downloadUrl,
+    context.downloadFilename || 'download',
+    context.downloadLocalPath || undefined
+  );
+
+  if (context.outputFileIds && result.savedPath) {
+    for (const fileId of context.outputFileIds) {
+      context.markSaved(fileId as FileId, result.savedPath);
+    }
+  }
+
+  return result;
+}

frontend/src/core/services/pdfExportService.ts

@@ -1,4 +1,5 @@
 import { PDFDocument as PDFLibDocument, degrees, PageSizes } from 'pdf-lib';
+import { downloadFile } from '@app/services/downloadService';
 import { PDFDocument, PDFPage } from '@app/types/pageEditor';
 
 export interface ExportOptions {
@@ -181,18 +182,7 @@ export class PDFExportService {
    * Download a single file
    */
   downloadFile(blob: Blob, filename: string): void {
-    const url = URL.createObjectURL(blob);
-    const link = document.createElement('a');
-    link.href = url;
-    link.download = filename;
-    link.style.display = 'none';
-
-    document.body.appendChild(link);
-    link.click();
-    document.body.removeChild(link);
-
-    // Clean up the URL after a short delay
-    setTimeout(() => URL.revokeObjectURL(url), 1000);
+    void downloadFile({ data: blob, filename });
   }
 
   /**