**Problem:**
When downloading Excel/xlsx (and other binary files like PDF, PPT, Word)
from the preview panel in WebUI mode, the downloaded file is always 0KB.

**Root Cause:**
Two compounding issues:

1. When a binary file is opened in the preview panel, `content` is set to
   an empty string `""` (since binary files cannot be loaded as UTF-8 text).
   The preview components use `metadata.filePath` instead. The download
   handler however did `new Blob([content])`, producing an empty 0-byte blob.

2. The naive fix of using `ipcBridge.fs.readFileBuffer` doesn't work in
   WebUI mode because the WebSocket bridge serializes ArrayBuffer to a
   UTF-8 string, corrupting binary data. Using `fetch(dataUrl)` on a
   `data:` URL is also blocked by CSP (`connect-src` does not allow `data:`).

**Fix:**
Add a dedicated branch in `handleDownload` for binary content types
(excel, pdf, ppt, word) when `metadata.filePath` is available:

- Use `ipcBridge.fs.getImageBase64` to read the file as a base64-encoded
  data URL (this IPC already exists, transfers as a plain JSON string,
  and is safe over WebSocket — no binary corruption).
- Decode the base64 in-memory using `atob()` + `Uint8Array` (no `fetch()`
  call, so no CSP violation).
- Create a `Blob` with the correct MIME type for the file extension.

This approach works in both Electron desktop and WebUI modes.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>