Add explicit Save As button for desktop viewer (issue #5928) (#5959)

## Description

Adds an explicit **“Save As”** button to the desktop viewer so users can
always save a copy of the current PDF to a different location, even if
the original file already has a local path.

This complements the existing smart **Save/Download** behavior:
- The existing download button continues to either save back to the
original path (when available) or prompt for a path when needed.
- The new **Save As** button always opens a save dialog to choose a
location/name for a new copy.

## Changes

- **RightRail (viewer controls)**
- Added a new **Save As** action icon in the right rail settings
section.
  - The button:
- Uses `viewerContext.exportActions.saveAsCopy()` to get the current
viewer state as a PDF.
- Calls `downloadFile` without a `localPath`, ensuring the desktop app
shows a **Save As** dialog.
- Picks the first selected file (if any) or the first active file as the
source for the filename.
- **Desktop / Web behavior**
  - In the desktop app (Tauri), clicking **Save As**:
- Opens a native save dialog so the user can choose a different folder
and filename.
- Writes a new copy without changing the existing file’s `localFilePath`
or dirty state.
- In the web app, the button behaves like a standard download of a copy
(browser-controlled save dialog / download).

## Motivation

- Users often want to apply operations on a PDF while **keeping the
original unmodified**.
- The existing smart Save behavior chooses between Save and Save As
automatically, but there was no way to explicitly request **Save As**.
- This change gives desktop users a clear, dedicated **“Save As”**
control while preserving the current Save/Download behavior.

## Notes

- No backend changes.
- No changes to the existing Save / Download button behavior.
- The new button uses existing viewer export and download utilities,
minimizing new logic.

---------

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

frontend/src/core/components/shared/RightRail.tsx

@@ -134,68 +134,71 @@ export default function RightRail() {
     [actions, allButtonsDisabled, disableForFullscreen, tooltipPosition, tooltipOffset]
   );
 
-  const handleExportAll = useCallback(async () => {
-    if (currentView === 'viewer') {
-      const buffer = await viewerContext?.exportActions?.saveAsCopy?.();
-      if (!buffer) return;
-      const fileToExport = selectedFiles.length > 0 ? selectedFiles[0] : activeFiles[0];
-      if (!fileToExport) return;
-      const stub = isStirlingFile(fileToExport) ? selectors.getStirlingFileStub(fileToExport.fileId) : undefined;
-      try {
-        const result = await downloadFile({
-          data: new Blob([buffer], { type: 'application/pdf' }),
-          filename: fileToExport.name,
-          localPath: stub?.localFilePath,
-        });
-        if (!result.cancelled && stub && result.savedPath) {
-          fileActions.updateStirlingFileStub(stub.id, {
-            localFilePath: stub.localFilePath ?? result.savedPath,
-            isDirty: false,
-          });
-        }
-      } catch (error) {
-        console.error('[RightRail] Failed to export viewer file:', error);
-      }
-      return;
-    }
-
-    if (currentView === 'pageEditor') {
-      pageEditorFunctions?.onExportAll?.();
-      return;
-    }
-
-    const filesToExport = selectedFiles.length > 0 ? selectedFiles : activeFiles;
-
-    if (filesToExport.length > 0) {
-      for (const file of filesToExport) {
-        const stub = isStirlingFile(file) ? selectors.getStirlingFileStub(file.fileId) : undefined;
+  const handleExportAll = useCallback(
+    async (forceNewFile = false) => {
+      if (currentView === 'viewer') {
+        const buffer = await viewerContext?.exportActions?.saveAsCopy?.();
+        if (!buffer) return;
+        const fileToExport = selectedFiles.length > 0 ? selectedFiles[0] : activeFiles[0];
+        if (!fileToExport) return;
+        const stub = isStirlingFile(fileToExport) ? selectors.getStirlingFileStub(fileToExport.fileId) : undefined;
         try {
           const result = await downloadFile({
-            data: file,
-            filename: file.name,
-            localPath: stub?.localFilePath
+            data: new Blob([buffer], { type: 'application/pdf' }),
+            filename: fileToExport.name,
+            localPath: forceNewFile ? undefined : stub?.localFilePath,
           });
-          if (result.cancelled) continue;
-          if (stub && result.savedPath) {
+          if (!forceNewFile && !result.cancelled && stub && result.savedPath) {
             fileActions.updateStirlingFileStub(stub.id, {
               localFilePath: stub.localFilePath ?? result.savedPath,
-              isDirty: false
+              isDirty: false,
             });
           }
         } catch (error) {
-          console.error('[RightRail] Failed to export file:', file.name, error);
+          console.error('[RightRail] Failed to export viewer file:', error);
+        }
+        return;
+      }
+
+      if (currentView === 'pageEditor') {
+        pageEditorFunctions?.onExportAll?.();
+        return;
+      }
+
+      const filesToExport = selectedFiles.length > 0 ? selectedFiles : activeFiles;
+
+      if (filesToExport.length > 0) {
+        for (const file of filesToExport) {
+          const stub = isStirlingFile(file) ? selectors.getStirlingFileStub(file.fileId) : undefined;
+          try {
+            const result = await downloadFile({
+              data: file,
+              filename: file.name,
+              localPath: forceNewFile ? undefined : stub?.localFilePath,
+            });
+            if (result.cancelled) continue;
+            if (!forceNewFile && stub && result.savedPath) {
+              fileActions.updateStirlingFileStub(stub.id, {
+                localFilePath: stub.localFilePath ?? result.savedPath,
+                isDirty: false,
+              });
+            }
+          } catch (error) {
+            console.error('[RightRail] Failed to export file:', file.name, error);
+          }
         }
       }
-    }
-  }, [
-    currentView,
-    selectedFiles,
-    activeFiles,
-    pageEditorFunctions,
-    viewerContext,
-    selectors,
-    fileActions,
-  ]);
+    },
+    [
+      currentView,
+      selectedFiles,
+      activeFiles,
+      pageEditorFunctions,
+      viewerContext,
+      selectors,
+      fileActions,
+    ]
+  );
 
   const downloadTooltip = useMemo(() => {
     if (currentView === 'pageEditor') {
@@ -264,7 +267,7 @@ export default function RightRail() {
               variant="subtle"
               radius="md"
               className="right-rail-icon"
-              onClick={handleExportAll}
+              onClick={() => handleExportAll()}
               disabled={
                 disableForFullscreen ||
                 (currentView !== 'viewer' && (totalItems === 0 || allButtonsDisabled))
@@ -276,6 +279,24 @@ export default function RightRail() {
             tooltipPosition,
             tooltipOffset
           )}
+          {icons.saveAsIconName &&
+            renderWithTooltip(
+              <ActionIcon
+                variant="subtle"
+                radius="md"
+                className="right-rail-icon"
+                onClick={() => handleExportAll(true)}
+                disabled={
+                  disableForFullscreen ||
+                  (currentView !== 'viewer' && (totalItems === 0 || allButtonsDisabled))
+                }
+              >
+                <LocalIcon icon={icons.saveAsIconName} width="1.5rem" height="1.5rem" />
+              </ActionIcon>,
+              t('rightRail.saveAs', 'Save As'),
+              tooltipPosition,
+              tooltipOffset
+            )}
         </div>
 
         <div className="right-rail-spacer" />

frontend/src/core/hooks/useFileActionIcons.ts

@@ -2,8 +2,9 @@ import UploadIcon from '@mui/icons-material/Upload';
 import DownloadOutlinedIcon from '@mui/icons-material/DownloadOutlined';
 
 /**
- * File action icons for web builds
- * Desktop builds override this with different icons
+ * File action icons for web builds.
+ * Desktop builds override this file via TypeScript path aliases to provide
+ * different icons (e.g. Save icon instead of Download, and a Save As icon).
  */
 export function useFileActionIcons() {
   return {
@@ -11,5 +12,8 @@ export function useFileActionIcons() {
     download: DownloadOutlinedIcon,
     uploadIconName: 'upload' as const,
     downloadIconName: 'download' as const,
+    // Web builds do not expose a Save As icon — the button is hidden when this is undefined.
+    // Desktop builds override this file and return a real icon name.
+    saveAsIconName: undefined as string | undefined,
   };
 }

frontend/src/desktop/hooks/useFileActionIcons.ts

@@ -2,8 +2,9 @@ import FolderOpenOutlinedIcon from '@mui/icons-material/FolderOpenOutlined';
 import SaveOutlinedIcon from '@mui/icons-material/SaveOutlined';
 
 /**
- * File action icons for desktop builds
- * Overrides core implementation with desktop-appropriate icons
+ * File action icons for desktop builds.
+ * Overrides the core implementation with desktop-appropriate icons.
+ * The presence of `saveAsIconName` signals RightRail to show the Save As button.
  */
 export function useFileActionIcons() {
   return {
@@ -11,5 +12,8 @@ export function useFileActionIcons() {
     download: SaveOutlinedIcon,
     uploadIconName: 'folder-rounded' as const,
     downloadIconName: 'save-rounded' as const,
+    // Returning this icon name causes RightRail to render the Save As button.
+    // On desktop, downloadFile() without a localPath shows a native save dialog.
+    saveAsIconName: 'save-as-rounded' as string | undefined,
   };
 }