web: count a roll as it is read, and open a frame without losing the count

Two holes left by the last change. A frame is opened with `window.location.href`
rather than a link, so the studio was still handed a fresh page and the scan with
it — every route out of the catalogue now goes through `go()`, which pushes the
address instead of reloading while a scan is in flight. And a row counted what was
filed away rather than what had been read, so it sat at zero for the whole of a
first scan: the catalogue's frames only reach IndexedDB in one batch at the end.

`ScanProgress.counts` carries the frames the scan has reached, per row, off the
scan's own bookkeeping, and the column reads it while the scan runs — the row
under the reader's eye moves as the roll is read, and the toolbar's line stays the
whole picture. A re-read counts from the start, which is what it is doing.

The regression check grows the two: a frame opened mid-scan from the catalogue
(of a roll it already holds) has to stay in the page, and the row has to count.

docker/frontend/src/nav.ts

@@ -0,0 +1,16 @@
+import { scanSession } from './engine/library';
+
+// Internal navigation, in one place. The app has no router: a route is a page
+// load, and that is fine for everything except a route change made while a scan
+// is reading a roll, because the reading lives in this page's memory and a reload
+// throws it away. So while a scan is in flight the route changes in place —
+// push the address, tell whoever draws it — and with no scan running the browser
+// navigates exactly as it always has.
+export function go(url: string): void {
+  if (!scanSession()) {
+    window.location.href = url;
+    return;
+  }
+  history.pushState(null, '', url);
+  window.dispatchEvent(new PopStateEvent('popstate'));
+}