Add configurable external editor with keyboard shortcut

[myrison]

Summary

• Adds an "Open in External Editor" menu item that opens the current markdown file in a user-configured editor application
• Default shortcut is Cmd+E, configurable to any modifier+key combination in Settings
• No editor is configured by default — on first use, the user is prompted to select one
• Includes a custom shortcut recorder that captures modifier+key combinations with validation against reserved system shortcuts
• Ships with unit tests for the settings store

Details

• ExternalEditorSettings: Persists editor app URL, display name, and shortcut to UserDefaults
• SettingsView: Editor picker (scoped to /Applications) with icon preview, plus an interactive shortcut recorder
• ShortcutRecorderView: Captures keyboard events, validates against reserved system shortcuts (Cmd+Q, Cmd+W, etc.), and filters to ASCII-printable keys
• Dynamic menu: Menu item title and shortcut update reactively when settings change

Works with any application that accepts file URLs — tested with Sublime Text, VS Code, and BBEdit.

Screenshots

How to test

• Open Settings > External Editor, select an editor app
• Verify the menu item updates to show "Open in {EditorName}"
• Record a custom keyboard shortcut, verify it appears in the menu
• Open a markdown file and trigger the shortcut — editor should open the file
• Clear the editor selection, verify fallback to a file chooser prompt
• Run swift test — unit tests pass for ExternalEditorSettings

Follow-up

I'm planning a couple of smaller follow-up PRs as independent improvements that complement this workflow:

• Auto-reload with notification: Replace the "File changed" click-to-reload banner with automatic reload and a dismissable notice
• Bring to front on file change: Optional setting to raise the viewer window when an external edit is detected

Each will be submitted separately against main so they can be evaluated independently.

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