fix(files): review round 3, the page — a search waits for the server, the viewer keeps its file, a drop ends the drag

- past the list's cap a search waits in a skeleton, never showing the newest files' 'no match' for a
  file the server is about to find; a sort keeps its stand-in rows
- the viewer keeps the file it was opened on when a server answer replaces the rows under it
- a drop another zone took still clears the page's drop overlay
- 'The first N items' only for a server sort; the past-the-end notice no longer claims the file exists
- a failed server search is tested to say so

frontend/src/i18n/locales/en/files.json

@@ -11,7 +11,7 @@
   "folderUnknown": "This folder could not be loaded",
   "arrival": {
     "gone": "That file is no longer in your library",
-    "notListed": "That file is past the end of this list — search for it by name"
+    "notListed": "That file isn't among the ones listed here — search for it by name"
   },
   "phoneOk": "Plays on a phone too",
   "rail": {

frontend/src/i18n/locales/hu/files.json

@@ -11,7 +11,7 @@
   "folderUnknown": "Ez a mappa nem tölthető be",
   "arrival": {
     "gone": "Ez a fájl már nincs a könyvtáradban",
-    "notListed": "Ez a fájl kimaradt a listából — keress rá a nevére"
+    "notListed": "Ez a fájl nincs a listázottak között — keress rá a nevére"
   },
   "phoneOk": "Telefonon is lejátszható",
   "rail": {