Show place details beside the search field

Picking a search result fills a column on the left of the add-place dialog -
pictures and a description, no extra click. Someone who already knows the place
ignores it and saves exactly as before.

The column stays mounted with an empty state instead of appearing on the first
pick, otherwise the dialog jumps sideways mid-typing. Requests are cached in
memory and in sessionStorage and aborted when the selection changes, so clicking
back and forth between two results does not pay twice.

Every thumbnail carries its author, its licence and a link to the file page.
That is not decoration - most of these are CC BY-SA, and reusing one without
naming the author does not satisfy the licence. The description is shown but
never written into the place on its own; a button copies it across, and it is
disabled while the user already has a description of their own.

Desktop only. The mobile sheet is untouched.

client/src/components/shared/Modal.tsx

@@ -9,6 +9,10 @@ const sizeClasses: Record<string, string> = {
   xl: 'max-w-2xl',
   '2xl': 'max-w-4xl',
   '3xl': 'max-w-5xl',
+  // Wide enough for the add-place dialog to carry a detail column beside the
+  // form, and both together beside the collection picker.
+  '4xl': 'max-w-6xl',
+  '5xl': 'max-w-7xl',
 }
 
 interface ModalProps {