feat: add better tab view

docs/api.md

@@ -55,7 +55,10 @@ and `docs/server-stock-source-of-truth.md`.
 - `events` (at most 1000): terminal-generated UUID `id`, `type`, `occurredAt` (ISO-8601, UTC; capped at the server
   time when more than 5 minutes ahead) and per type:
   - `tab_opened` (`tabId`, `customerName`, `openedAt`), `tab_renamed` (new `customerName`), `tab_deleted`
-  - `line_added` / `line_removed`: `lineId`, `productId`, `productName`, `unitPriceInCents`, `quantity` (≥ 1)
+  - `line_added` / `line_removed`: `lineId`, `productId`, `productName`, `unitPriceInCents`, `quantity` (≥ 1) and
+    `orderId`: the order on the terminal the drinks belong to. Every unit keeps the `orderId` of its `line_added`, and
+    the dashboard and terminals show a tab per order. `line_removed` with an `orderId` (or `null`: drinks without an
+    order) only takes that order's drinks off; without the field (older app versions) the latest of the line
   - `settled`: `amountInCents`, `paymentMethod` (`cash`, `payconiq`, `payment_pending` or `other`), optional
     `commandId` and `items` (`[{ productId?, productName, quantity, unitPriceInCents }]`)
   - `stock_set`: `productId`, `quantity` (a count ≥ 0, or `null` to stop tracking); no `tabId`