Rename dropdown to menu, add combobox built on top of menu (#42202)

* Rename Dropdown component to Menu

Replace the Dropdown component with Menu across the entire codebase.
All .dropdown-* classes are now .menu-* classes, data-bs-toggle="dropdown"
is now data-bs-toggle="menu", and the JS export is renamed from Dropdown
to Menu. Includes updated tests, docs, examples, site UI, and config.

* Add new Combobox component

Introduce a searchable select component built on top of Menu, with
single and multi-select support. Includes JS source, SCSS styles,
documentation, and migration notes.

* Add translucent variations for navbar, card, toast, and menu

New .navbar-translucent, .card-translucent, and .toast-translucent
styles with backdrop blur and saturation filters, plus corresponding
documentation sections.

* Update docs site nav, layout, icons, and misc fixes

Clean up docs site navbar SCSS (sticky shadow, remove commented code),
fix empty line rendering in code snippets, add font-size class to
masthead, and add #stack icon symbol.

* Rebuild dist files