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 <james@stirlingpdf.com>

frontend/package.json

@@ -40,6 +40,7 @@
     "@mantine/hooks": "^8.3.1",
     "@mui/icons-material": "^7.3.2",
     "@mui/material": "^7.3.2",
+    "@posthog/react": "^1.8.2",
     "@reactour/tour": "^3.8.0",
     "@stripe/react-stripe-js": "^4.0.2",
     "@stripe/stripe-js": "^7.9.0",
@@ -63,7 +64,6 @@
     "license-report": "^6.8.0",
     "pdfjs-dist": "^5.4.149",
     "peerjs": "^1.5.5",
-    "@posthog/react": "^1.8.2",
     "posthog-js": "^1.268.0",
     "qrcode.react": "^4.2.0",
     "react": "^19.1.1",
@@ -174,8 +174,8 @@
     "postcss-preset-mantine": "^1.18.0",
     "postcss-simple-vars": "^7.0.1",
     "puppeteer": "^24.25.0",
+    "tsx": "^4.21.0",
     "typescript": "^5.9.2",
-    "tsx": "^4.19.4",
     "typescript-eslint": "^8.44.1",
     "vite": "^7.1.7",
     "vite-plugin-static-copy": "^3.1.4",