Combobox shell: header input + anchored results popover

Make the header bar the real search input and show results in a popover anchored under it, over a backdrop scrim, replacing the centered NcDialog. Wire it to the existing UnifiedSearchService for real results. The scrim + popover render in the header itself, not a teleported dialog, so the input stays visible above the dimmed page; NcModal with empty container gives the scrim, focus-trap and Esc without teleporting.

• header bar converted from <button> to a real input
• results render in the anchored popover with real data, not a centered dialog
• backdrop scrim (rendered inside #header, not teleported) dims the page; clicking the scrim or Esc closes; focus trapped while open
• static combobox ARIA: input role=combobox + aria-expanded/aria-controls; keep our own roles on the panel, not role=dialog
• in-field loading-spinner element present
• plugin API + nextcloud:unified-search:* events still work through the swap