Show a printer's camera, from the backend's address not a build-time one

`WebRTCStreamView` has existed since before this commit with no consumer:
nothing rendered it, and the URL it would have used came from a
`go2rtcStreamBase` in `nuxt.config.ts` defaulting to
`http://localhost:1984`.

That default was the real problem. A camera stream lives on a gateway box
inside one farm's network, each farm has its own, and the gateway now
reports its address to the backend when it connects. A compiled-in
constant could only ever be correct for a single-farm install, and its
localhost default pointed every player at the operator's own laptop. It
is gone, replaced by `fetchPrinterStream`, which asks
`GET /printers/:id/stream`.

Absence is the normal case here, not an error: only Bambu's X1/H2 class
publishes a camera URL a gateway can restream, so a P1 or A1 has a camera
and nothing to connect to. The backend 404s for those, this resolves to
null, and the card renders no player - offering one that can never load
would be worse than offering nothing. The lookup also runs only for
printers whose `cameraStream` the fleet list already reports, so a
twenty-printer farm does not make twenty requests to learn that eighteen
of them 404.

`PrinterCard` takes the stream as a prop rather than fetching it, like
`live` above it: `packages/ui` is plain Vue that knows nothing about the
API or about Nuxt, and that is what lets these components render from a
test or the desktop shell with no server behind them.

Three things in `WebRTCStreamView` that would have bitten whoever first
pointed it at a real printer:

- The fetch status was never checked. go2rtc answers a request for a
  stream it does not have with an error page, and handing that to
  `setRemoteDescription` throws about SDP parsing - which sends you
  looking at the wrong layer entirely.
- `connect()` is async inside a watcher with nothing catching it, so a
  failure left the badge on "connecting" forever and showed up only as an
  unhandled rejection in the console.
- Two prop changes in quick succession raced: the slower request could
  finish last and apply its answer to a connection already replaced and
  closed, leaving a black video with a "connected" badge.

No `iceServers`, deliberately and now commented: both ends are on the
same LAN, so host candidates are enough. The same fact is the MVP's
limitation - there is no relay, so this plays for someone on the
printers' network and for nobody else.

`nuxt typecheck` and `vue-tsc --noEmit` both clean.

apps/web/composables/usePrinterStream.ts

@@ -0,0 +1,56 @@
+/**
+ * Where a printer's camera can be watched, from `GET /printers/:id/stream`.
+ */
+export interface PrinterStream {
+  streamUrl: string;
+  cameraId: string;
+}
+
+/**
+ * Asks the backend where a printer's camera can be watched, or `null` if it
+ * cannot.
+ *
+ * This replaces the build-time `go2rtcStreamBase` that used to live in
+ * `nuxt.config.ts`. A single compiled-in URL can only ever be correct for a
+ * one-farm deployment: the stream lives on a gateway box inside a farm's own
+ * network, each farm has its own, and the gateway reports its address to the
+ * backend when it connects. So the address has to be asked for at runtime,
+ * per printer, rather than baked in.
+ *
+ * A plain function and not a `useAsyncData` wrapper despite the file it lives
+ * in: the caller needs one of these per printer, discovered from a list that
+ * itself arrives asynchronously, and Nuxt's data composables may only be
+ * called while a component is setting up. A loop over a resolved list is not
+ * that.
+ *
+ * **Absence is the normal case, not an error.** Only Bambu's X1/H2 class
+ * publishes a camera URL a gateway can restream; a P1 or A1 reports that it
+ * has a camera and gives nothing to connect to. The backend answers 404 for
+ * those, so this answers `null` and the caller shows no player at all -
+ * rendering one that can never load would be worse than rendering nothing.
+ *
+ * It is also a LAN address with no relay in front of it, so the stream plays
+ * only for someone on the same network as the printers. That is the right
+ * trade-off for an operator standing in the workshop, and a real limitation
+ * for anyone else.
+ */
+export async function fetchPrinterStream(
+  apiBase: string,
+  printerId: string,
+  headers: Record<string, string>,
+): Promise<PrinterStream | null> {
+  try {
+    return await $fetch<PrinterStream>(`${apiBase}/printers/${printerId}/stream`, { headers });
+  } catch (err: unknown) {
+    // A 404 means "this printer has no watchable camera", which is expected
+    // often enough that treating it as a failure would put an error state on
+    // most cards in a mixed fleet. Anything else is a real problem: logged
+    // and then treated as "no stream", because a broken camera lookup should
+    // not take the fleet overview down with it.
+    const status = (err as { statusCode?: number }).statusCode;
+    if (status !== 404) {
+      console.warn(`could not look up the camera stream for printer ${printerId}`, err);
+    }
+    return null;
+  }
+}