feat(files): folders managed in place, a grid per folder, drop to upload, open on one file

- Own folders are made, renamed and deleted from the rail (right click, long press,
  Shift+F10) or the "…" beside an open folder's name; any folder's retention is a
  submenu (7/30/90 days, until deleted). Deleting confirms where its files go.
- List or grid per folder, sharing one sort; the grid is the one media card.
- A file dropped anywhere on the page opens Upload with it, bound for the folder
  on screen.
- A link (?item=) or the upload notice opens Files on the file: its page turned
  to, the keyboard on it, a ring round it.
- An in-place name field left by Enter or Escape hands the focus back to what
  replaced it (the row, the entry, the "…") instead of dropping it on <body>.
- The table's saved page size is per account.

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

@@ -103,6 +103,12 @@
       "move": "Couldn't move the file",
       "keep": "Couldn't change whether the file is kept",
       "delete": "Couldn't delete the file"
+    },
+    "folder": {
+      "create": "Couldn't create the folder",
+      "rename": "Couldn't rename the folder",
+      "retention": "Couldn't change how long the folder keeps its files",
+      "delete": "Couldn't delete the folder"
     }
   },
   "ok": "OK",