Fix toolbar separator visibility not updating when setting changes

[ivandrofly]

Summary

• Store toolbar top separator in ToolbarTopSeparator VM property instead of conditionally adding it
• Always add the separator to the visual tree but control visibility via IsVisible
• Update separator visibility in the settings-apply method so toggling the setting takes effect immediately without restart

Test plan

• Open Settings > Appearance, toggle "Show horizontal line above toolbar" on → separator appears above toolbar
• Toggle it off → separator disappears immediately
• Restart the app → setting is preserved correctly

Visual demo

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