web: a catalogue of the folders on the visitor's own disk

A RAW studio that cannot see a folder is one photo at a time. /library
now takes a folder through Chromium's directory picker, keeps the handle
in IndexedDB so the folder is there on the next visit, and walks it into
a grid: one thumbnail per frame, the frame's own date, and the recipe it
was last graded with. Nothing is uploaded and nothing is read twice —
the RAW itself is opened only when a tile is clicked, at which point the
studio develops it and the recipe comes back on top. The studio files
every change back against the frame, debounced, so reopening a RAW is
not doing the grade again.

Thumbnails come off LibRaw's unpack_thumb for a RAW, which is a seek and
a copy where a develop is a full decode of every pixel, and off
createImageBitmap for anything else. A RAW with no preview inside it
gets a placeholder tile rather than a minute of decoding per file.

  node scripts/library-check.mjs
  ok  both frames indexed as tiles — 2 tiles from P1010256.JPG + P1010256.RW2
  ok  thumbnail for P1010256.JPG — 40400 bytes, jpeg=true
  ok  thumbnail for P1010256.RW2 — 39895 bytes, jpeg=true      (LibRaw preview)
  ok  studio developed the frame from its handle
  ok  the address was handed back — url=/app (no ?lib= left behind)
  ok  the look was filed back against the frame — baseFilter=none, 19 knobs
  ok  the tile says the frame is edited

docker/frontend/src/engine/rawDevelop.ts

@@ -215,6 +215,27 @@ async function cameraPreview(raw: LibRaw): Promise<Uint8Array | null> {
   return new Uint8Array(thumb.data);
 }
 
+// The preview on its own, for the catalogue: a folder of RAW files has to show
+// a tile per frame, and unpack_thumb is a seek and a copy where the develop
+// above is a full decode of every pixel at full resolution. No preview inside
+// the file means no tile — the row still lists the frame by name, and the
+// studio develops it the moment it is opened.
+//
+// ponytail: still a whole LibRaw open per file, reading the file's bytes into
+// memory to do it. Fine for the scan's one-at-a-time walk; give the catalogue a
+// worker and a sync-access handle if a 10k-frame folder ever turns up.
+export async function rawThumbnail(bytes: Uint8Array): Promise<Uint8Array | null> {
+  const raw = new LibRaw();
+  try {
+    await raw.open(bytes as unknown as BufferSource, SETTINGS);
+    return await cameraPreview(raw);
+  } catch {
+    return null;
+  } finally {
+    raw.dispose();
+  }
+}
+
 // MATCH_GRID x MATCH_GRID block colours of a frame, one byte per channel.
 function gridOf(image: any, n = MATCH_GRID): Uint8Array | null {
   const surface = Skia.Surface.MakeOffscreen(n, n) ?? Skia.Surface.Make(n, n);