Record why the camera player cannot reach a gateway directly

Measured against go2rtc 1.9.14: it sends no Access-Control-Allow-Origin
on any response, auth or not; the SDP POST needs a preflight anyway
because application/sdp is not a CORS-safelisted Content-Type; and
go2rtc checks basic auth before CORS, so the preflight - which browsers
always send without credentials - gets a 401.

So this component has never worked from a browser on another origin, and
no credential handed to the frontend could fix it. The gateway's API now
requires a password, which closes the hole where anything on the
workshop LAN could watch, and changes the symptom here from a CORS error
to a 401. Both are the same missing piece: the relay.

Also stops the two most misleading errors in this path. A blocked
cross-origin request rejects as an opaque TypeError ("Failed to fetch")
that reads as the gateway being down, and a 401 reads as a wrong URL or
a missing camera. Both now say what they are.

packages/ui/src/components/WebRTCStreamView.vue

@@ -16,9 +16,28 @@ const error = ref<string | null>(null);
  *
  * Note what is deliberately absent: no `iceServers`. Both ends are on the
  * same LAN - the gateway is a box in the workshop - so host candidates are
- * enough and a STUN round-trip would only add latency. The same fact is why
- * this component cannot work from outside that network at all; there is no
- * relay, and the backend's /printers/:id/stream says so too.
+ * enough and a STUN round-trip would only add latency.
+ *
+ * ## This cannot work from a browser on another origin, and that is measured
+ *
+ * Not a caveat about reachability - a flat blocker, against go2rtc 1.9.14:
+ *
+ * - go2rtc sends **no** `Access-Control-Allow-Origin` on any response, with
+ *   or without auth configured. A cross-origin response is unreadable.
+ * - The `POST` below needs a preflight regardless, because `application/sdp`
+ *   is not a CORS-safelisted `Content-Type`.
+ * - go2rtc checks basic auth *before* CORS, so the preflight - which a
+ *   browser always sends without credentials - gets a 401. Sending the
+ *   gateway's password from here could not fix it even if the password were
+ *   worth handing to every viewer.
+ *
+ * So this component works when it is same-origin with go2rtc, or through a
+ * relay that proxies server-to-server. The gateway's API now requires a
+ * password (continuum-proxy's camera.rs), which closes the hole where
+ * anything on the workshop LAN could watch - and which also means a browser
+ * pointed straight at a gateway now gets a 401 rather than a CORS error.
+ * Both are the same missing piece. The relay design is in PLAN.md; note it
+ * cannot carry WebRTC, so whatever lands there will replace this transport.
  */
 async function connect() {
   status.value = "connecting";
@@ -53,6 +72,15 @@ async function connect() {
   // whoever is debugging it looking at the wrong layer, when the real
   // answer is that this printer is not in go2rtc's config.
   if (!response.ok) {
+    // 401 has one meaning and it is worth naming: go2rtc's API is
+    // password-protected, and a browser cannot send that password across
+    // origins (see above). It is not a wrong URL or a missing camera.
+    if (response.status === 401) {
+      throw new Error(
+        "the gateway requires a password for camera access, which a browser cannot " +
+          "supply across origins - this needs the relay",
+      );
+    }
     throw new Error(`the gateway refused the stream request (HTTP ${response.status})`);
   }
 
@@ -76,7 +104,18 @@ watch(
   () => {
     connect().catch((err: unknown) => {
       status.value = "failed";
-      error.value = err instanceof Error ? err.message : String(err);
+      // A blocked cross-origin request rejects as an opaque TypeError
+      // ("Failed to fetch") with nothing about CORS in it, which is the
+      // single most misleading error in this path - it reads as the gateway
+      // being down. Say the likely cause instead of repeating the browser.
+      error.value =
+        err instanceof TypeError
+          ? "could not reach the gateway's camera API from this origin - either it is " +
+            "unreachable from here, or the browser blocked the request (see this " +
+            "component's notes)"
+          : err instanceof Error
+            ? err.message
+            : String(err);
     });
   },
   { immediate: true },