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/pages/index.vue

@@ -7,6 +7,12 @@ interface Printer {
   name: string;
   model: string;
   farmId: string;
+  /**
+   * Set only for a printer whose camera the gateway can actually restream,
+   * which in a mixed fleet is the minority. Its presence is what decides
+   * whether to ask for a stream URL at all - see the lookup below.
+   */
+  cameraStream?: string | null;
 }
 
 const telemetry = useTelemetryStore();
@@ -15,6 +21,33 @@ const config = useRuntimeConfig();
 const { data: printers } = await useFetch<Printer[]>(`${config.public.apiBase}/printers`, {
   headers: useRequestHeaders(["cookie"]),
 });
+
+// Looked up per printer, because the answer is per printer, but only for the
+// printers the list already says have a camera. Asking for all of them would
+// mean one request per machine in the fleet, most of them 404 by design.
+const streams = ref<Record<string, PrinterStream | null>>({});
+
+// Read here, in setup, rather than inside the loop below: Nuxt's request
+// helpers are only valid while a component is setting up.
+const streamHeaders = useRequestHeaders(["cookie"]) as Record<string, string>;
+
+watch(
+  printers,
+  (list) => {
+    for (const printer of list ?? []) {
+      if (!printer.cameraStream || printer.id in streams.value) continue;
+      // Marked before the request resolves, so a re-render in the meantime
+      // does not fire a second lookup for the same printer. `null` renders
+      // no player, which is also the right thing to show while this is in
+      // flight.
+      streams.value[printer.id] = null;
+      void fetchPrinterStream(config.public.apiBase, printer.id, streamHeaders).then((stream) => {
+        streams.value[printer.id] = stream;
+      });
+    }
+  },
+  { immediate: true },
+);
 </script>
 
 <template>
@@ -26,6 +59,7 @@ const { data: printers } = await useFetch<Printer[]>(`${config.public.apiBase}/p
         :key="printer.id"
         :printer="printer"
         :live="telemetry.printers[printer.id]"
+        :stream="streams[printer.id]"
       />
     </div>
     <p v-if="!printers?.length">No printers registered yet.</p>