Caspian Store

## v12.0.0 — Register correctness pass, and the till is a PWA only

A full review of `src/pos/` turned up sixteen defects, from a receipt printed with no
number on it to a real sale that could sit invisible in the outbox forever. All of them
are fixed here. Separately, the Tauri desktop shell is gone: the register ships as a PWA
and only as a PWA.

### Consumer action required on upgrade

`commitPosSale` now returns the priced lines it already computes, and the register builds
the customer's receipt from those. Redeploy the `caspian-pos` codebase or receipts keep
falling back to the till's scanned prices:

```bash
npm install github:CaspianTools/script-caspian-store#v12.0.0
cd firebase/functions-pos && npm install && npm run build && cd ../..
firebase deploy --only functions:caspian-pos
```

Nothing else changes for a consumer site. If your code imported `readStorageMode`,
`writeStorageMode`, `readPrinterTransport` or `writePrinterTransport`, delete those calls —
see **Removed**.

### Fixed

- **An offline sale with no leased numbers left printed a blank receipt number.**
  `PosSaleQueue.capture` returns a device-local reference for exactly this case and
  `PosQueuedCloudAdapter` was discarding it, so the customer got a slip with no identifier
  at all. The reference is now used and labelled as provisional, on screen and on the
  printed receipt.
- **A sale interrupted mid-send was never retried and never cleared.** `send()` marks a
  sale `sending` before the call; a reload or a closed tab left it there permanently —
  `drain()` only picks up `held`, `pruneSent()` only removes `sent`, and the held-sales
  page offers Retry on `blocked` alone. The queue now sweeps `sending` back to `held` once
  per session, before the online check so the count is right even offline.
- **Receipt lines could disagree with the receipt total.** Lines and subtotal came from the
  open ticket while the total came back from the commit, so a catalogue edit mid-sale
  printed a slip whose own lines did not add up to its own total. `commitSale` now returns
  what was charged and the receipt is built from that.
- **An online sale burnt a leased receipt number the server never honoured.**
  `PosCloudAdapter` was not forwarding the `receipt` claim (or `capturedByUid` /
  `capturedByName`) to the callable, so the ordinal was spent locally while the server
  allocated a different number from the counter. Leases drained at one number per sale and
  numbering developed permanent gaps.
- **Typing into the register's quick-add dialogs fired phantom barcode scans.** The
  keyboard wedge was silenced only for the tender phase, which could not see the product
  and person dialogs the POS header opens outside `PosRegister`. It now detects any open
  dialog from the DOM, so consumer-added dialogs are covered too.
- **`1,234.50` was read as `1.234` at the tender screen.** `String.replace` with a string
  argument replaces only the first match, so a grouping separator produced two decimal
  points and `parseFloat` stopped at the second. On the tendered field that is wrong change
  handed to a customer.
- **The register, the connection pill and the held-sales page each built their own
  outbox.** They shared IndexedDB so no sale was lost, but `capture()` and `markSent()`
  notified an instance nobody was subscribed to, so the held-sales badge only moved on a
  30-second timer and `paused` — in-memory state — could never be observed by the pill.
  There is now one adapter, provided by `PosShell`.
- **Backups silently dropped custom roles.** `LOCAL_BACKUP_VERSION` is 2 and carries
  `roles`; v1 files still restore. `parseLocalBackup` also validates `sales` is an array,
  which it never did while `restoreLocalBackup` iterated it.
- **"0 receipt numbers left" could never be cleared.** The low-numbers warning fired on
  `leasedRemaining === 0`, which is also a healthy till before its first lease and a store
  with no `functions-pos` deployment. Lease failure is now tracked separately and reported
  as "Cannot reserve receipt numbers".
- The `Math.random` fallback in `randomId()` had no length floor, and the server requires a
  sale id of at least eight characters.

### Added

- **Line discounts have a control at last.** `usePosTicket.setLineDiscount` was plumbed
  end to end — cloud payload, `priceLocalSale`, `commitPosSale` validation, receipt
  renderer — with nothing calling it, so the ticket's discount row and the receipt's
  per-line discount were unreachable. Each ticket line now has a markdown button.
- `PosAdapterProvider` / `usePosAdapter`, the shared register storage seam.
- `PosSoldLine` — a line as it was charged, distinct from the scanned `PosSaleLine`.
- `summariseSoldLines` and `parseAmount` are exported, both pure and both checked by
  `scripts/check-standalone.mjs`.

### Removed

- **`desktop/` and its build workflow.** The Tauri shell that bundled the register for
  Windows is gone; the till is installed from Chrome or Edge with the register's own
  Install button. Existing `desktop/v*` releases stay on GitHub but are no longer built or
  documented, and the POS manual's Windows-installer section has been deleted. Standalone
  mode itself (`<CaspianStoreProvider standalone>`, `PosLocalAdapter`, the local back
  office) is untouched and still exported — it simply has no first-party packaged delivery.
- `readStorageMode`, `writeStorageMode`, `readPrinterTransport`, `writePrinterTransport`
  and `idbCountByIndex`. Nothing called any of them: the printer control is hard-coded to
  the browser transport until the ESC/POS paths ship, and the storage mode is derived by
  `resolvePosStorageMode` rather than stored.
- `promoCode` from the standalone commit path. `priceLocalSale` never applied a promo, so
  the field recorded a claim nothing honoured.

### Changed

- `PosQueuedCloudAdapter` takes its queue as a constructor argument instead of building one.
- `PosCommittedSale` gains optional `lines` and `provisionalReceipt`.