Dropdown Menu

Elevate the Nordstar visual language with a shared depth and motion system, then apply it across the components.

• New design tokens shipped from @nordcom/nordstar's CSS contract:
• Elevation — shadow-raised, shadow-overlay, shadow-floating, a three-step shadow scale tuned to read on the near-black default surface.
• Easing — ease-out-soft (hard deceleration for entrances) and ease-spring (a slight overshoot for toggles and presses).
• Motion — an overlay-in keyframe / animate-overlay-in utility used by portalled surfaces, plus accordion-down/accordion-up for disclosure height transitions. The existing prefers-reduced-motion reset neutralises them for users who opt out.
• Overlay surfaces (Select, DropdownMenu, Tooltip) now lift on the new shadow-overlay, animate in from their transform origin, and round to md. The Select chevron rotates while the listbox is open.
• `Button` gains a resting elevation on the solid variant, a subtle hover lift that settles on press, and a consistent keyboard focus ring.
• `Card` solid surfaces read as raised via shadow-raised.
• `Switch` thumb slides on the springy easing with a touch of depth.
• `Kbd` picks up a recessed keycap shadow.
• `Accordion` sections now expand and collapse with a measured height animation instead of snapping open.
• Motion timings across Input, Select, and the dropdown items were aligned to the shared duration/easing language.

No public APIs changed; these are visual and motion refinements within the existing design identity.

Add <DropdownMenu/>, a compound action menu built on @radix-ui/react-dropdown-menu (Trigger, Content, Item, CheckboxItem, RadioGroup, RadioItem, Label, Separator, Group, Sub, SubTrigger, SubContent). It ships the full menu role, roving focus, typeahead, and keyboard/pointer dismissal, with check/dot/chevron indicators inlined (no icon dependency) and the surface re-fitted to the Nordstar token contract.