feat: add animated context menu

[starc007]

What changed

• add a composable animated context menu with item, checkbox, radio, label, separator, and shortcut primitives
• support right-click, long-press, keyboard invocation, typeahead, collision-aware portal positioning, and reduced motion
• use a pointer-origin clip morph with matching 300ms open and close motion
• add a focused folder preview and register the component at @beui/context-menu
• add repeated-open and accessibility regression coverage
• update the sidebar lifetime offer copy to $199

Why

The library needed a production-ready context menu that feels native, remains reliable across repeated invocations, and ships through the existing registry contract. The stable inert portal avoids the earlier mount/exit race that could prevent later right-clicks from opening the menu.

Validation

• bun run check
• bun test — 57 tests passing
• registry validation — 54 components