Make vehicle bed use mattress instead of seat

[ampersand55]

Describe the bug

Currently you install a vehicle bed using a seat, but it would be more realistic if you installed it from a mattress you get from disassembling a furniture bed.

I'd do it myself, but I vaguely remember there was something about tests for vehicle weights that needed updating when changing the mass of its parts.

For reference: a mattress weighs 20 kg and a seat weighs 9.07 kg.

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