feat(mgmt/web): surface the host.env encoder pin so a conflicting GPU choice isn't invisible

listGpus gains encoder_pin (PUNKTFUNK_ENCODER when it actually pins something —
unset/auto stay null), and the console's GPU card shows it: a muted note when
the pin is compatible, an amber warning naming the pinned vendor and the next
session's GPU when it contradicts the selection (the host overrides such a pin
at session open — without this field the selection just looked broken). Docs
updated to say the adapter wins.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/mgmt/tests.rs

@@ -1234,6 +1234,13 @@ async fn gpu_endpoints_list_and_validate() {
     assert_eq!(s, StatusCode::OK);
     assert!(b["gpus"].is_array());
     assert!(b["mode"].is_string());
+    // The host.env encoder pin is part of the schema (null when nothing is pinned) — the
+    // console warns off it when a pin contradicts the selected GPU (the pin is overridden at
+    // session open, and without this field the selection would just look broken).
+    assert!(
+        b.as_object().unwrap().contains_key("encoder_pin"),
+        "listGpus must carry encoder_pin"
+    );
 
     // Unknown mode → 400.
     let (s, _) = send(