style(motion): introduce --duration-base for panel-class transitions

Adds 0.24s between --duration-fast (0.15s) and --duration-slow (0.4s)
for animations that live in the touch-UI floor: panels, sheets,
drawers, modals. 150ms reads as crisp on desktop but undershoots the
200-300ms range touch UI conventions prescribe (finger occludes the
target during the tap, deliberation pace is slower, and high-DPI
displays compress small translateY into less perceptible motion).

Applies the new token to the mobile hamburger panel open/close
transition and to the bar-morph that fires on the same tap, so the
bars and the panel finish arriving together.

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