fix(library): land a reading's rows as they are read, and keep the copy the reader reads offline

- scan: rows go down on a clock as well as on a full batch. The reader's folder
  of 26 RAWs is under one batch of 50, so nothing of it reached the screen until
  the reading was over — a wall that stood empty for the whole scan and then
  filled in one go. Frames now reach the screen every 400ms while the roll is
  still being read, so the count climbs and the first tiles are drawn as the
  frames land.
- tiles: a HEIC or a RAW has its tile written the first time it is drawn, so
  every later visit reads the tile back instead of the file.
- resume: a reading a reload cut off is offered by the next visit with a chip
  instead of being run behind the reader's back.
- offline listing: the desk's listing is kept whole in IndexedDB under
  `recipescam-offline`/`reads`, past the 200 000 characters localStorage held,
  so an offline visit opens on the listing it kept instead of on nothing.
- offline writes: a vote or a view cast with no server is held and goes out when
  one answers again, with a bar that says how many are waiting.
- sw: the shell precaches its own bundle, stylesheet and icon, read out of the
  document — a worker that precached only the document answered an offline
  launch with a page whose JavaScript the machine did not have.
- checks in docker/frontend/scripts: tree-scan-check (counts while reading,
  resume after a reload, tiles of a long roll), offline-read-check,
  offline-queue-check.

docker/frontend/src/ui/OfflineBar.tsx

@@ -0,0 +1,34 @@
+import { useEffect, useState } from 'react';
+import { useI18n } from '../i18n/I18nProvider';
+import { heldCount, isReachable, subscribe } from '../pwa/offline';
+
+// The server cannot be reached. A bar, not a dialog: nothing is being asked of
+// the visitor, and the answer — the saved copy of what the screen was holding —
+// is already on the screen. It goes away by itself the moment a call answers.
+//
+// It also stays up while writes are still held: the server being back is not the
+// same thing as the server having them, and a queue that drained quietly would
+// leave the visitor wondering whether the last change landed. See `hold`/`flush`
+// in pwa/offline.ts.
+export function OfflineBar() {
+  const { t } = useI18n();
+  const [gone, setGone] = useState(() => !isReachable());
+  const [held, setHeld] = useState(heldCount);
+
+  useEffect(
+    () =>
+      subscribe(() => {
+        setGone(!isReachable());
+        setHeld(heldCount());
+      }),
+    []
+  );
+
+  if (!gone && held === 0) return null;
+  const lines = [gone ? t('offline.notice') : '', held > 0 ? t('offline.pending', { n: held }) : ''];
+  return (
+    <div className="offline-bar" role="status" data-key="offline-bar">
+      {lines.filter(Boolean).join(' ')}
+    </div>
+  );
+}