Make the Windows/Linux menu bar collapsible behind the brand

[danipen]

Summary

• Turn the always-visible Windows/Linux custom menu bar into a collapsible one, toggled by the brand button (with a folded-in chevron) instead of taking up title-bar space by default.
• Persist the expanded/collapsed state in localStorage (gg.menuExpanded), mirroring how the theme preference is stored. Defaults to collapsed so the title bar stays tidy.
• MenuBar now takes an expanded prop and renders null while collapsed, staying mounted so the labels fetched on mount survive toggles without a refetch flicker.
• On macOS the brand still opens About (no chevron); on Windows/Linux About moves to the Help menu and the brand doubles as the menu toggle.

Details

• Toolbar.tsx: owns menuExpanded state + toggleMenu, wires the brand button to toggle (or About on macOS), passes expanded down to MenuBar.
• MenuBar.tsx: accepts expanded, stays mounted while collapsed.
• global.css: styles for the chevron inside the brand (muted color, rotates between collapsed/expanded), tweaked brand padding.

Test plan

• On Windows/Linux: brand starts collapsed; clicking it expands the menu bar and the chevron rotates; state persists across reloads.
• On macOS: brand still opens About, no chevron shown, native menu unchanged.

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