feat(nav): restore scroll-hide and animate mobile hamburger

Three changes to the mobile floating header:

- Restore the desktop scroll-driven hide/show on the corner button by
  dropping the transform:none override. Override the translate distance
  under 800px so the toggle (44px tall at top:1.25rem) and its 5px
  outline ring fully clear the viewport: translateY(calc(-100% - 2rem))
  instead of -150%, which had been leaving 3px of ring poking through.

- Replace display:none / display:flex on the open panel (uneanimatable)
  with always-rendered panel that flips opacity, transform, visibility,
  and pointer-events. Uses --duration-fast / --ease-out-quart so the
  motion vocabulary matches the rest of the project. transform-origin
  is top right so the scale grows from the toggle position.

- Move per-item styling out of the .menu-open qualifier so items keep
  their block layout even while the panel is faded out, avoiding a
  layout reflow at the moment menu-open flips.

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