web-ui: keep the device safe-area under the composer on mid-width surfaces (#491)

On mid-width surfaces (tablets and panes between the phone breakpoint and full width), the floating composer and live-work dock hugged the column with fixed 16px side margins and an 18px bottom margin, ignoring device safe-area insets — on notched or home-indicator devices the composer sat under the system UI. Give those margins an inset floor (max of the design margin and 10px plus the corresponding env(safe-area-inset-*)), matching the treatment the narrowest breakpoint already had.

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