Fix PDF file handling in Peek preview

[Copilot]

Issue

Peek was not displaying PDF files correctly. Users reported that Peek would become "out of order" when previewing PDF files, resulting in an unusable preview experience.

Root Cause

PDF files were being handled by the WebBrowserPreviewer component, which isn't optimized for displaying PDF documents. The WebBrowserPreviewer was intercepting PDF file preview requests before the more capable ShellPreviewHandlerPreviewer could handle them.

Solution

This PR makes a minimal change to fix the issue:

• Removed .pdf extension from WebBrowserPreviewer's supported file types list

With this change, when a user attempts to preview a PDF file in Peek:

1. WebBrowserPreviewer will no longer claim to support PDF files
2. The preview request will flow to ShellPreviewHandlerPreviewer
3. ShellPreviewHandlerPreviewer will use the system's native PDF preview handler (typically from Adobe Reader, Microsoft Edge, or another installed PDF viewer)

This ensures that PDF files are displayed properly using the system's registered PDF preview handler rather than attempting to render them with a component not designed for this purpose.

Fixes #11.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• cdn.fwupd.org
  • Triggering command: /usr/bin/fwupdmgr refresh (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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