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/files.json

@@ -56,5 +56,35 @@
     "unkeep": "Stop keeping",
     "details": "Details…",
     "upload": "Upload"
-  }
+  },
+  "folder": {
+    "menu": "Folder actions",
+    "new": "New folder",
+    "newName": "Folder name",
+    "rename": "Rename",
+    "retention": "Keeps files",
+    "forever": "Until I delete them",
+    "delete": "Delete folder…",
+    "deleteTitle": "Delete “{{name}}”?",
+    "deleteBodyEmpty": "The folder is empty. Nothing else changes.",
+    "deleteBody_one": "Its file goes back to the folder it came from, and its retention starts again from today.",
+    "deleteBody_other": "Its {{count}} files go back to the folders they came from, and their retention starts again from today.",
+    "deleteAction": "Delete folder"
+  },
+  "layout": {
+    "label": "View",
+    "list": "List",
+    "grid": "Grid"
+  },
+  "sort": {
+    "label": "Sort",
+    "default": "Newest first",
+    "asc": "Ascending",
+    "desc": "Descending"
+  },
+  "drop": {
+    "here": "Drop to upload into {{folder}}",
+    "oneAtATime": "Upload one file at a time — the first one is ready to go."
+  },
+  "perPage": "Files per page"
 }