Add optional bring-to-front on file change

[myrison]

Summary

• Adds a View menu toggle ("Bring to Front on File Change") that raises the viewer window when the monitored file is modified externally
• Off by default — opt-in behavior
• Minimal change: 2 files, 9 lines added

Details

• DocumentState: Checks UserDefaults in the file monitor event handler and calls NSApplication.shared.activate(ignoringOtherApps: true) when enabled
• MarkdownViewerApp: Adds an @AppStorage-backed toggle under View > Zoom controls

Note

If #13 is also merged, this toggle can be consolidated into the Settings modal for a cleaner UI. As a standalone, the View menu toggle is fully self-contained.

Companion to #13 — as noted in that PR's follow-up section — but independent and submitted against main.

How to test

• Open a .md file in the viewer
• Enable View > Bring to Front on File Change
• Switch to another app, edit the file externally and save
• Verify the viewer window comes to front
• Disable the toggle, repeat — viewer should stay in background
• Verify the setting persists across app restarts

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