Fix duplicate React keys in the file-assign place list

When a place is assigned to the same day more than once it appeared twice in a day's list, so the place-button key={p.id} collided and React warned about duplicate keys. Key by place id + render index so siblings stay unique. Pre-existing in the old FileManager; behaviour unchanged.

client/src/components/Files/FileManagerAssignModal.tsx

@@ -64,10 +64,10 @@ export function AssignModal(S: FileManagerState) {
               }
             }
             const unassigned = places.filter(p => !assignedPlaceIds.has(p.id))
-            const placeBtn = (p: Place) => {
+            const placeBtn = (p: Place, idx: number) => {
               const isLinked = file.place_id === p.id || (file.linked_place_ids || []).includes(p.id)
               return (
-                <button key={p.id} onClick={async () => {
+                <button key={`${p.id}-${idx}`} onClick={async () => {
                   if (isLinked) {
                     if (file.place_id === p.id) {
                       await handleAssign(file.id, { place_id: null })