Extract useClickOutside hook, dedupe popover close-on-outside-click logic

RegexHelpIcon and LogPreview's ColumnPicker each reimplemented the same
mousedown-outside-ref-closes-popover effect; share it via a small hook.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

webapp/src/lib/useClickOutside.ts

@@ -0,0 +1,14 @@
+import { useEffect, type RefObject } from 'react'
+
+// Shared by popover-style UI (RegexHelpIcon, LogPreview's ColumnPicker) that closes itself when a
+// mousedown lands outside its container ref, while `active`.
+export function useClickOutside(ref: RefObject<HTMLElement | null>, onOutside: () => void, active: boolean) {
+  useEffect(() => {
+    if (!active) return
+    function onClickOutside(e: MouseEvent) {
+      if (ref.current && !ref.current.contains(e.target as Node)) onOutside()
+    }
+    document.addEventListener('mousedown', onClickOutside)
+    return () => document.removeEventListener('mousedown', onClickOutside)
+  }, [active, ref, onOutside])
+}