Combobox shell: header input + anchored results popover

[pringelmann]

Make the header bar the real search input and render results in a popover anchored under it (over a backdrop scrim), replacing the centered NcDialog.

No single @nextcloud/vue component fits, NcModal/NcDialog are centered, NcPopover has no scrim, so build a small custom anchored-overlay: an anchored panel (CSS or floating-ui), a backdrop <div>, the @nextcloud/vue focus-trap util, and combobox/listbox ARIA.

• header bar converted from <button> to a real input
• results render in the anchored popover, not a centered dialog
• backdrop scrim dims the page; clicking the scrim or Esc closes; focus trapped while open
• in-field loading-spinner element present
• usable at 320px / mobile