FileBrowser Vulnerable to Stored XSS via SVG File in Public Share (Missing CSP Header)

Summary

## Summary

FileBrowser Quantum serves inline SVG files without a `Content-Security-Policy` header, allowing embedded JavaScript in SVG files to execute when accessed via public share links.

Verified on v1.3.0-stable.

## Affected product

- **Product:** FileBrowser Quantum (`gtsteffaniak/filebrowser`) - **Verified version:** v1.3.0-stable - **Docker image:** gtstef/filebrowser:latest - **Affected endpoint:** `GET /public/api/resources/download?hash=HASH&inline=true` - **CWE:** CWE-79 — Cross-site Scripting (Stored)

## Impact

- **Stored XSS** — Malicious SVG persists and executes for every visitor to the share link - **No authentication required to trigger** — Public share links are accessible to anyone - **Session hijacking** — If authenticated users click the link, their session can be stolen - **Phishing** — Attacker can redirect or overlay fake login forms

## Reproduction

1. Login as any user with upload permission 2. Upload SVG file: ```xml <svg xmlns="http://www.w3.org/2000/svg"> <script>alert(document.domain)</script> </svg> ``` 3. Create public share for the file 4. Access the share link with `?inline=true` 5. JavaScript executes in browser

## Root cause

The inline download endpoint returns SVG files with: ``` Content-Type: image/svg+xml Content-Disposition: inline; filename="xss.svg" X-Content-Type-Options: nosniff ```

But no CSP header to block script execution. The upstream project (filebrowser/filebrowser) mitigates this with: ``` Content-Security-Policy: script-src 'none' ```

## Suggested fix

Add CSP header on inline file downloads:

```go w.Header().Set("Content-Security-Policy", "script-src 'none'") ```

This matches the upstream filebrowser/filebrowser implementation.