Keyboard navigation + Cmd/Ctrl+K

[pringelmann]

Add the combobox keyboard model (the interaction the new search is missing) and the global shortcut. Port the proven logic from LegacyUnifiedSearch.vue (focusNext/focusPrev, onInputEnter, auto-focus first).

• first result auto-selected on render; up/down move the selection via aria-activedescendant while the input keeps focus; Enter opens the selected result
• a global shortcut (Ctrl+K or Alt+K, decide per issue) focuses the input without clobbering app shortcuts; Esc closes
• aria-live announces searching / done / result counts (WCAG 4.1.3)
• logical focus order + visible focus on the selected result (2.4.3 / 2.4.7)
• verified with a screen reader