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/components/ui/pressableSelectable.contract.test.ts

@@ -17,7 +17,8 @@ const files = readSources(__dirname, [
 
 /** file → [how many plain Pressables, why they are not titles]. */
 const PLAIN: Record<string, [number, string]> = {
-  "FilesRail.tsx": [1, "the load-error retry"],
+  "FileCard.tsx": [1, "a card's picture (named by aria-label; its title is the caption)"],
+  "FilesRail.tsx": [2, "the load-error retry; the New-folder command"],
   "Home.tsx": [
     2,
     "the hero's picture and a shelf tile's picture (each named by aria-label; its title sits outside)",