fix(app): keep the app on its feet — a boundary, one window, and a reading that ends

- boundary: src/ui/ErrorBoundary.tsx, around the whole app rather than around a
  screen. A throw on the way to the first paint used to be a white page with
  nothing in it to report; it is a sentence, the message and the stack now, with
  a reload beside them, so the next report of a crash carries what went wrong.

- one window: src/ui/SingleTab.tsx holds the app to one tab over localStorage and
  a BroadcastChannel. A second tab on /app or /library is told so and offered the
  roll — "use this tab" takes it, and the tab that held it gets a bar and the
  offer back. A tab holding a reading is a tab the machine has already spent its
  memory on, and a second one beside it is the memory the first will not get.

- mount: the studio and the library are mounted the first time their route is
  asked for and stay mounted after that, not on every visit. Booting CanvasKit
  (~20MB) and reading the whole catalogue on a page that shows neither is the
  read and the memory a machine short of either takes for a crash; a trip back
  from the studio is a screen opening rather than a page loading.

- reading: READ_MS bounds one touch of the disk at 30s. getFile and arrayBuffer
  on a drive that has gone away do not fail, they never settle — the lane waits
  for ever, the reading never ends, and STOP is a button with nothing left to
  read it. A frame given up on is a frame the next reading takes up again.

- reading: the rows of a walk go down a pass at a time, so a tab that never comes
  back leaves the column with the folders this reading walked rather than the ones
  the last reading did.

- reading: a stopped reading hands over the numbers it counted instead of null.
  Handed null it threw its own count away and had noughts written for every folder
  it walked that the file did not already hold, so a folder added to a drive read
  nought until the next scan.

- repair: STOP ends the afternoon. A repair is a loop the button has no handle on,
  so it holds the count of stops it started at and compares between rolls, and
  says it was cut short rather than reporting a run it did not make.

- add: a folder the browser could not name — every drive root on Windows is "\" —
  is asked for one, and a folder whose name is already another folder's is refused
  rather than folded in with it. The record, the frame ids, the walk's position
  and the number the column draws are all keyed by that name.

- wall: four hundred tiles queued ahead is a reader waiting on frames three
  screens below the one they are looking at; sixty is a screenful and the wall
  mints the rest as it scrolls. The catalogue is read back every 30s, not every
  5s — reading it back costs every thumbnail in it.

- library: the note is drawn under the frame rather than above it. A line that
  comes and goes in the flow moved the frame, the column and the strip by its own
  height every time a scan started or ended.

- manifest: "id": "/", so the installed app and the tab are one app.

- checks: tree-scan-check waited for a count of exactly four rows — a state the
  screen does not have. The wait ran out its clock and then measured whatever the
  reading had done since; it waits for the whole tree now.

docker/frontend/scripts/tree-scan-check.mjs

@@ -281,11 +281,16 @@ await page.evaluate(() => localStorage.setItem('check.hold', '1'));
 await page.click('[data-key="lib-add"]');
 await page.waitForSelector('[data-key="lib-node-CheckRoll"]', { timeout: 30_000 });
 // Every row of the pass is a row now, and a count beside each name is what the
-// walk met — the reading itself has read nothing.
+// walk met — the reading itself has read nothing. The whole tree is drawn in one
+// go and never a row at a time, so the wait is for all of it: a count in between
+// is a state this screen does not have, and waiting for one is a wait that runs
+// out its clock and then measures whatever the reading has done since. The
+// reading is held on the frame it is reading, and how long a held frame stays
+// held is the engine's to decide (`READ_MS`), not this file's.
 await page
   .waitForFunction(() => {
     const rows = [...document.querySelectorAll('[data-key^="lib-node-"]')];
-    return rows.length === 4 && /\d+\/\d+/.test(document.querySelector('[data-key="lib-progress"]')?.textContent ?? '');
+    return rows.length >= 5 && /\d+\/\d+/.test(document.querySelector('[data-key="lib-progress"]')?.textContent ?? '');
   }, null, { timeout: 60_000, polling: 50 })
   .catch(() => {});
 const midFloat = await counts();