Linux: 'Show in File Explorer' displays macOS Security dialog instead of opening file manager

## Description

On Linux, clicking "Show in File Explorer" in the Keybase GUI Files section displays a **macOS-specific dialog** asking to "Open your macOS Security & Privacy Settings" instead of opening the native file manager.

## Steps to Reproduce

1. Open Keybase GUI on Linux
2. Navigate to **Files** section
3. Click "Yes, enable" on the "Enable Keybase in File Explorer?" prompt
4. Or click on "Show in File Explorer" icon next to any folder

## Expected Behavior

The native Linux file manager (Nautilus, Dolphin, etc.) should open at the selected KBFS path.

## Actual Behavior

A dialog appears with **macOS-specific instructions**:
- Title: "You need to change your system security preferences."
- Text: "Open your macOS Security & Privacy Settings and follow these steps."
- Shows macOS Security preferences UI
- Has link "Open Security & Privacy Settings"

This is clearly platform-inappropriate for Linux systems.

## Environment

- **OS:** Pop!_OS 22.04 LTS (Ubuntu-based)
- **Kernel:** 6.17.x
- **Keybase Version:** 6.5.1
- **Desktop Environment:** GNOME
- **File Manager:** Nautilus

## Workaround

```bash
nautilus /keybase
# or
xdg-open /keybase/private/username
```

## Related Issues

- #13238 - Client will not open my local file explorer to browse kbfs files (open, 2018)
- #17098 - Unable to show Files in File Explorer option on Linux (closed as regression, 2019)

This appears to be a regression where the Linux code path now incorrectly falls through to macOS-specific UI.

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345567

Start by reproducing the issue from the GUI Files section using the “Show in File Explorer” action on Linux, and compare it with `xdg-open /keybase` or Nautilus. Trace the platform-specific handling for the file-manager prompt and verify that Linux opens the selected KBFS path without showing macOS instructions.

Written by the indexing model from the issue text.