Extract a shared combobox hook for SearchBox and TagPicker

Follow-up from PR #155 (ARIA correctness).

`SearchBox` and `TagPicker` now both implement the ARIA APG combobox-with-listbox pattern by hand: `open` / `activeIndex` state, clamped `activeIdx`, `useId()`-derived listbox/option ids, `aria-activedescendant`, wrapping ArrowUp/ArrowDown, Enter/Escape, guarded `onMouseMove` highlight, and close-on-focus-out. The two copies have already drifted in small ways (SearchBox swallows mousedown on the popup and closes on input blur; TagPicker uses a container `onBlur` + `relatedTarget` check with `tabIndex={-1}` options).

Extract a `useCombobox` hook (or adopt shadcn's `Command`) so the keyboard/ARIA contract lives in one place and the two widgets only differ in how they render options and what activation does. Keep the existing `SearchBox.test.tsx` / `TagPicker.test.tsx` as the behavioural contract.

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https://claude.ai/code/session_01RdRwHvDupRLV8GuJpYKzEr