Show File Manager label on Linux for the Open Folder button

[ivan-ottinger]

Related issues

• Fixes STU-1753

How AI was used in this PR

Used Claude Code to locate the two label sites (the site-overview shortcut button and the site context menu) and add the Linux branch. I verified the changes, lint, typecheck, and ran the affected tests myself.

Proposed Changes

• On Linux, show "File Manager" (instead of falling through to the macOS "Finder" label) for the Open Folder shortcut button on the site overview tab.
• Same fix for the "Open in …" item in the site list context menu.

Testing Instructions

• On Linux, open Studio and select a site.
• On the Overview tab, in the "Open in…" section, confirm the first button is labeled File Manager (was "Finder").
• Right-click a site in the sidebar and confirm the menu shows Open in File Manager (was "Open in Finder").
• Clicking either still opens the site folder in the system file manager.
• Verify macOS still shows Finder and Windows still shows File Explorer.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?