Keyboard navigation + Cmd/Ctrl+K

[pringelmann]

Add the combobox keyboard model and the global shortcut. Port the proven logic from LegacyUnifiedSearch.vue.

• first result auto-selected on render; up/down move the selection while the input keeps focus; Enter opens the selected result
• a global shortcut (Ctrl+K or Alt+K, TBC) focuses the input without clobbering app shortcuts; Esc closes
• aria-live announces searching / done / result counts
• logical focus order + visible focus on the selected result
• verified with a screen reader