fix(SearchPalette): return null while closed

The Overlay refactor dropped the early return, so the fixed-inset backdrop
stayed mounted permanently: the palette covered the page and swallowed
every click. Restore the guard and cover it with tests.

src/components/SearchPalette.tsx

@@ -197,6 +197,11 @@ export function SearchPalette({ isOpen, onClose, onSelect, initialQuery = '' }:
     trapTabKey(e, panelRef.current)
   }
 
+  /* Closed palette must not render at all: the backdrop is `position: fixed;
+     inset: 0`, so rendering it unconditionally left the modal permanently on
+     screen (and ate every click on the page underneath). */
+  if (!isOpen) return null
+
   return (
     <div className="search-palette-backdrop" onClick={onClose}>
       <div