feat: add widgets sidebar toggle button to view menu (#3140)

Add ability to toggle the Widgets sidebar visibility via a button in the
tabbar. State persists across sessions and workspaces through workspace
metadata (layout:widgetsvisible).

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: sawka <mike@commandline.dev>