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

@@ -4475,6 +4475,7 @@ exitRedaction = "Exit Redaction Mode"
 save = "Save"
 downloadAll = "Download All"
 saveAll = "Save All"
+saveAs = "Save As"
 
 [textAlign]
 left = "Left"