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/ui/TopBar.tsx

@@ -98,6 +98,9 @@ export function TopBar({
       </button>
       <span className="spacer" />
 
+      {/* The catalogue is a page of its own, open to guests as much as to
+          members — it reads the visitor's own folders and never the server. */}
+      <a className="btn ghost" data-key="nav-library" href="/library">{t('nav.library')}</a>
       <button type="button" className="btn ghost" onClick={onReset}>{t('act.reset')}</button>
       <button
         type="button"