feat(library): give a folder its missing thumbnails, and the wall its stars

Four asks, one screen: a folder can be given the tiles it is short of, by hand
or on the way in; the rescan row says RESCAN...; and the wall of thumbnails
scores a frame without the trip to the stage.

- Library.tsx + i18n: "RESCAN FROM THE TOP" is "RESCAN..." in both languages.
  The reading is unchanged — a row's roll is still read from the top, its
  subfolders included.
- engine/library.ts + Library.tsx: the tile of one frame is one function now
  (`makeThumb`: the store, the backup behind it, then the file's own preview
  for a RAW or a HEIC and a decode for anything else). The wall's lazy path
  calls it instead of keeping its own copy of the same branches.
- engine/library.ts: `missingThumbs` asks which of a folder's rows no tile was
  ever drawn for, in one transaction, and `makeThumbs` makes those — a few
  frames at a time, and giving up when the caller has moved on.
- Library.tsx: the folder menu carries MAKE THUMBS, which reads the row's
  folder and what lies under it; and opening a folder asks for its own frames'
  tiles by itself, silently, in the background — the branch too when the
  screen is drawing the branch. A frame that lands after the ask is asked for
  again, which is why the frame count is a dependency of the effect.
- Library.tsx: a wall card carries the same five stars the stage does, under
  the picture, and rates the frame on the click. The click stops at the star,
  so scoring a frame is not picking it.

Verified in Chrome at 1440x900 against OPFS standing in for the disk, three
frames with one of them in a subfolder: opening the folder leaves two tiles
(the node's own frames) and MAKE THUMBS takes the roll to three, the frame in
the subfolder included; clearing the store and reloading brings the two back
on the open; the menu reads RESCAN... and MAKE THUMBS; and the wall's fourth
star lights four, writes star 4 to the catalogue row, and takes it back on the
second click. The same checks pass on the built bundle at :8090.

docker/frontend/src/i18n/en.ts

@@ -248,12 +248,14 @@ export const en: Dict = {
   'lib.rename': 'RENAME FOLDER',
   'lib.renameAsk': 'Name shown for this folder (the folder on the disk is not renamed):',
   'lib.update': 'UPDATE...',
-  'lib.rescan': 'RESCAN FROM THE TOP',
+  'lib.rescan': 'RESCAN...',
+  'lib.makeThumbs': 'MAKE THUMBS',
+  'lib.thumbsMade': 'Made {n} thumbnails for {folder}.',
   'lib.resume': 'KEEP READING {folder}',
   'lib.resumeHint': 'The last reading of this folder was cut off. Press to read it on from where it stopped.',
   'lib.remove': 'REMOVE',
   'lib.reconnect': 'GRANT ACCESS AGAIN',
-  'lib.nodeHint': 'Right-click to rename, update, rescan, collapse or remove the folder.',
+  'lib.nodeHint': 'Right-click to rename, update, rescan, make the missing thumbnails, collapse or remove the folder.',
   'lib.collapseAll': 'COLLAPSE ALL',
   'lib.collapse': 'COLLAPSE',
   'lib.colHint': 'Right-click the empty part of this column to add a folder.',